Last visit was: 19 Jul 2025, 13:12 It is currently 19 Jul 2025, 13:12
Close
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Close
Request Expert Reply
Confirm Cancel
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 19 Jul 2025
Posts: 102,625
Own Kudos:
Given Kudos: 98,235
Products:
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 102,625
Kudos: 742,773
 [37]
4
Kudos
Add Kudos
33
Bookmarks
Bookmark this Post
Most Helpful Reply
User avatar
pushpitkc
Joined: 26 Feb 2016
Last visit: 19 Feb 2025
Posts: 2,819
Own Kudos:
5,874
 [10]
Given Kudos: 47
Location: India
GPA: 3.12
Posts: 2,819
Kudos: 5,874
 [10]
7
Kudos
Add Kudos
3
Bookmarks
Bookmark this Post
User avatar
generis
User avatar
Senior SC Moderator
Joined: 22 May 2016
Last visit: 18 Jun 2022
Posts: 5,293
Own Kudos:
36,964
 [6]
Given Kudos: 9,464
Products:
Expert
Expert reply
Posts: 5,293
Kudos: 36,964
 [6]
3
Kudos
Add Kudos
3
Bookmarks
Bookmark this Post
General Discussion
User avatar
GMATinsight
User avatar
Major Poster
Joined: 08 Jul 2010
Last visit: 19 Jul 2025
Posts: 6,377
Own Kudos:
15,614
 [5]
Given Kudos: 128
Status:GMAT/GRE Tutor l Admission Consultant l On-Demand Course creator
Location: India
GMAT: QUANT+DI EXPERT
Schools: IIM (A) ISB '24
GMAT 1: 750 Q51 V41
WE:Education (Education)
Products:
Expert
Expert reply
Schools: IIM (A) ISB '24
GMAT 1: 750 Q51 V41
Posts: 6,377
Kudos: 15,614
 [5]
3
Kudos
Add Kudos
2
Bookmarks
Bookmark this Post
Bunuel
The cost of fuel increases by 10%. By what % must the consumption of fuel decrease to keep the overall amount spent on the fuel same?

(A) 5%
(B) 9%
(C) 10%
(D) 11%
(E) 20%

Let Initial cost per consumed unit of fuel = 100
Let Initial Consumption = 100

Initial Total cost = 100*100


New cost per consumed unit of fuel = 110
Let New Consumption = C

New Total cost = 110*C

now 100*100 = 110*C
i.e. C = 90.90 = 91 (approx)

% Decrease = 9%

ANswer: Option B
User avatar
mohshu
Joined: 21 Mar 2016
Last visit: 26 Dec 2019
Posts: 426
Own Kudos:
Given Kudos: 103
Products:
Posts: 426
Kudos: 133
Kudos
Add Kudos
Bookmarks
Bookmark this Post
assuming numbers will be best strategy to tackle this kind of problems...

let the cost be 10$ and the consumption be 10 litres,,
total cost is 100..
after the increase, the cost is 11$.. in 100$ we can purchase slightly more than 9 litres of fuel,,
hence required decrease in consumption shud be 9%

ans B
User avatar
Skywalker18
User avatar
Retired Moderator
Joined: 08 Dec 2013
Last visit: 15 Nov 2023
Posts: 2,052
Own Kudos:
9,704
 [1]
Given Kudos: 171
Status:Greatness begins beyond your comfort zone
Location: India
Concentration: General Management, Strategy
GPA: 3.2
WE:Information Technology (Consulting)
Products:
Posts: 2,052
Kudos: 9,704
 [1]
Kudos
Add Kudos
1
Bookmarks
Bookmark this Post
Bunuel
The cost of fuel increases by 10%. By what % must the consumption of fuel decrease to keep the overall amount spent on the fuel same?

(A) 5%
(B) 9%
(C) 10%
(D) 11%
(E) 20%

Let original cost of fuel = 100
total consumption = 10

New cost of fuel = 110
total consumption post fuel price increase = x
100*10 = 110*x
=> x = 1000/110 = 100/11 = 9 % approx

Answer B
User avatar
amanvermagmat
User avatar
Retired Moderator
Joined: 22 Aug 2013
Last visit: 28 Mar 2025
Posts: 1,161
Own Kudos:
Given Kudos: 480
Location: India
Posts: 1,161
Kudos: 2,745
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Amount spent = Cost * Consumption = P*Q (say)

Here we have to keep amount spent (the product of two quantities) same. Question is that if one quantity increases by 10%, then by how much the other must decrease so as to keep the product same (as P*Q only)

So if P increases by 10% (or 1/10) new value of P is = P + P/10 = 11P/10
To keep the product same, new value of Q must be = 10Q/11 (because then only 11P/10 * 10Q/11 = p*Q only)

Thus, Q needs to decrease by 1/11 or by 9.09%. Closest option is B, hence B answer
User avatar
JeffTargetTestPrep
User avatar
Target Test Prep Representative
Joined: 04 Mar 2011
Last visit: 05 Jan 2024
Posts: 2,996
Own Kudos:
Given Kudos: 1,646
Status:Head GMAT Instructor
Affiliations: Target Test Prep
Expert
Expert reply
Posts: 2,996
Kudos: 7,952
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Bunuel
The cost of fuel increases by 10%. By what % must the consumption of fuel decrease to keep the overall amount spent on the fuel same?

(A) 5%
(B) 9%
(C) 10%
(D) 11%
(E) 20%

We can let the original cost of fuel = x and the original consumption = y. Thus, the original cost = xy. If cost increases 10%, the new cost is 1.1x. We can let n = the percentage decrease and create the following equation:

xy = (1.1x)(y(1-n/100))

1 = (1.1)((100 - n)/100)

100 = (1.1)(100 - n)

100 = 110 - 1.1n

1.1n = 10

n = 10/1.1 ≈ 9 percent

Alternate Solution:

Let’s assume that fuel was $10 per gallon and we used 10 gallons, so our total spent was 10 x 10 = $100. The new cost of fuel is $11, but we are still spending $100. Letting x = the new amount of fuel we will use, we now have:

11x = 100

x = 100/11 = 9.09

We must decrease our fuel usage from 10 gallons to 9.09 gallons, which is a decrease of 0.91 gallons, or 0.91/10 = 9.1% decrease.

Answer: B
User avatar
Mislead
Joined: 03 Jan 2017
Last visit: 16 Jul 2025
Posts: 44
Own Kudos:
72
 [1]
Given Kudos: 48
Concentration: Finance, Economics
GMAT 1: 600 Q47 V27
GMAT 1: 600 Q47 V27
Posts: 44
Kudos: 72
 [1]
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
The problem can be solved just by looking at the question if you know the following rule:
if the price of a product increases by 1/x (%age converted into a fraction) here it is increased by 10% so the increase is 1/10 and we need to keep the total cost same as earlier so we would reduce the consumption of the product by 1/x+1 here it will be 1/11 which is 9.09%

Hence, Answer C
User avatar
KarishmaB
Joined: 16 Oct 2010
Last visit: 19 Jul 2025
Posts: 16,115
Own Kudos:
74,405
 [3]
Given Kudos: 475
Location: Pune, India
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 16,115
Kudos: 74,405
 [3]
3
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Bunuel
The cost of fuel increases by 10%. By what % must the consumption of fuel decrease to keep the overall amount spent on the fuel same?

(A) 5%
(B) 9%
(C) 10%
(D) 11%
(E) 20%

Responding to a pm:

Overall Expense = Cost per unit * number of units

If we need to keep overall expense same, while cost per unit increases, number of units should decrease.

We are given that cost per unit increases by 10% i.e. becomes x + (10/100)*x = (11/10)*x

So the first term of right hand side "Cost per unit" is multiplied by 11/10. To ensure that Overall Expense does not change, we should multiply the second term of right hand side, "number of units" by 10/11.

So new number of units should be 10/11 of the original number of units

New number of units = 10/11 * (number of units)

New number of units = (1 - 1/11) * Number on units

New number of units = Number on units - (1/11)*Number of units

In percentage terms, 1/11 = 9.09%

So new number of units is 9.09% less than original number of units.

Answer (B)
User avatar
HolaMaven
Joined: 12 Aug 2017
Last visit: 06 Mar 2021
Posts: 60
Own Kudos:
134
 [4]
Given Kudos: 13
Status:Math Tutor
GMAT 1: 750 Q50 V42
WE:Education (Education)
GMAT 1: 750 Q50 V42
Posts: 60
Kudos: 134
 [4]
1
Kudos
Add Kudos
3
Bookmarks
Bookmark this Post
Consider some example:
Increasing something by 25%, then by how much percent should the new value be decreased to get the original amount.
Convert 25% into equivalent fraction which is \(\frac{1}{4}\)
Since we are looking for % decrease, increase the denominator by the numerator value to get the required fraction and its equivalent percent. In this case, since increase is by \(\frac{1}{4}\), we need to decrease the value by \(\frac{1}{4+1}\) = \(\frac{1}{5}\) = 20% to get the original value.

Example II:
Decreasing something by 25%, then by how much percent should the new value be increased to get the original amount.
Convert 25% into equivalent fraction which is \(\frac{1}{4}\)
Since we are looking for % increase, decrease the denominator by the numerator value to get the required fraction and its equivalent percent. In this case, since decrease is by \(\frac{1}{4}\), we need to increase the value by \(\frac{1}{4-1}\) = \(\frac{1}{3}\) = 33.33% to get the original value.
User avatar
testcracker
Joined: 24 Mar 2015
Last visit: 02 Dec 2024
Posts: 202
Own Kudos:
126
 [1]
Given Kudos: 541
Status:love the club...
Posts: 202
Kudos: 126
 [1]
Kudos
Add Kudos
1
Bookmarks
Bookmark this Post
HolaMaven
Consider some example:
Increasing something by 25%, then by how much percent should the new value be decreased to get the original amount.
Convert 25% into equivalent fraction which is \(\frac{1}{4}\)
Since we are looking for % decrease, increase the denominator by the numerator value to get the required fraction and its equivalent percent. In this case, since increase is by \(\frac{1}{4}\), we need to decrease the value by \(\frac{1}{4+1}\) = \(\frac{1}{5}\) = 20% to get the original value.

Example II:
Decreasing something by 25%, then by how much percent should the new value be increased to get the original amount.
Convert 25% into equivalent fraction which is \(\frac{1}{4}\)
Since we are looking for % increase, decrease the denominator by the numerator value to get the required fraction and its equivalent percent. In this case, since decrease is by \(\frac{1}{4}\), we need to increase the value by \(\frac{1}{4-1}\) = \(\frac{1}{3}\) = 33.33% to get the original value.

Wow man, thanks ...
Does the below problem also fall under this category ....?

Recently, fuel price has seen a hike of 20%. Mr X is planning to buy a new car with better mileage as compared to his current car. By what % should the new mileage be more than the previous mileage to ensure that Mr X’s total fuel cost stays the same for the month? (assuming the distance traveled every month stays the same)
User avatar
HolaMaven
Joined: 12 Aug 2017
Last visit: 06 Mar 2021
Posts: 60
Own Kudos:
134
 [1]
Given Kudos: 13
Status:Math Tutor
GMAT 1: 750 Q50 V42
WE:Education (Education)
GMAT 1: 750 Q50 V42
Posts: 60
Kudos: 134
 [1]
Kudos
Add Kudos
1
Bookmarks
Bookmark this Post
gmatcracker2017
HolaMaven
Consider some example:
Increasing something by 25%, then by how much percent should the new value be decreased to get the original amount.
Convert 25% into equivalent fraction which is \(\frac{1}{4}\)
Since we are looking for % decrease, increase the denominator by the numerator value to get the required fraction and its equivalent percent. In this case, since increase is by \(\frac{1}{4}\), we need to decrease the value by \(\frac{1}{4+1}\) = \(\frac{1}{5}\) = 20% to get the original value.

Example II:
Decreasing something by 25%, then by how much percent should the new value be increased to get the original amount.
Convert 25% into equivalent fraction which is \(\frac{1}{4}\)
Since we are looking for % increase, decrease the denominator by the numerator value to get the required fraction and its equivalent percent. In this case, since decrease is by \(\frac{1}{4}\), we need to increase the value by \(\frac{1}{4-1}\) = \(\frac{1}{3}\) = 33.33% to get the original value.

Wow man, thanks ...
Does the below problem also fall under this category ....?

Recently, fuel price has seen a hike of 20%. Mr X is planning to buy a new car with better mileage as compared to his current car. By what % should the new mileage be more than the previous mileage to ensure that Mr X’s total fuel cost stays the same for the month? (assuming the distance traveled every month stays the same)


Since fuel price increased by 20%, the mileage also has to be increased by same percentage to keep the cost same.
This can be understood as-
Total Fuel required = \(\frac{Total Distance}{Mileage}\)
Total Cost = Price (per Lt.) * total liters of fuel required = Price (per Lt) *\(\frac{Total Distance}{Mileage}\)
Now since Total cost is constant and price is increased by \(\frac{1}{5}\), Mileage also needs to be increased by same percentage to keep the total cost constant.
User avatar
HolaMaven
Joined: 12 Aug 2017
Last visit: 06 Mar 2021
Posts: 60
Own Kudos:
Given Kudos: 13
Status:Math Tutor
GMAT 1: 750 Q50 V42
WE:Education (Education)
GMAT 1: 750 Q50 V42
Posts: 60
Kudos: 134
Kudos
Add Kudos
Bookmarks
Bookmark this Post
For fastening the calculation, some important percentage equivalent fractions which generally used in aptitude papers
100% = \(\frac{1}{1}\)
87.5% = \(\frac{7}{8}\)
80% = \(\frac{4}{5}\)
75% = \(\frac{3}{4}\)
66.66% = \(\frac{2}{3}\)
62.5% = \(\frac{5}{8}\)
60% = \(\frac{3}{5}\)
50% = \(\frac{1}{2}\)
40% = \(\frac{2}{5}\)
37.5% = \(\frac{3}{8}\)
33.33% = \(\frac{1}{3}\)
25% = \(\frac{1}{4}\)
20% = \(\frac{1}{5}\)
16.66% = \(\frac{1}{6}\)
14.28% = \(\frac{1}{7}\)
12.5% = \(\frac{1}{8}\)
11.11% = \(\frac{1}{9}\)
10% = \(\frac{1}{10}\)
9.09% = \(\frac{1}{11}\)
8.33% = \(\frac{1}{12}\)
5% = \(\frac{1}{20}\)
2% = \(\frac{1}{50}\)
1% = \(\frac{1}{100}\)
User avatar
Manasvi1
Joined: 16 Feb 2019
Last visit: 18 Sep 2022
Posts: 30
Own Kudos:
61
 [1]
Given Kudos: 20
Posts: 30
Kudos: 61
 [1]
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Let,
Cost of fuel=x and Consumption of fuel=y
Overall amount spent = x*y= xy

Now, Cost of fuel is increased by 10%
New cost of fuel = x + 10%of x
= x + 10/100x =110x/100
Let new consumption of fuel= y'
Now, Overall amount spent=(110x/100)*y'
According to question,
New Overall amount spent=Old Overall amount spent
=> (110x/100) * y' = xy
=> y' = xy/x*100/110
=> y' = 10y/11

Decrease = (y - y')/y = (y - 10y/11)/y = 1/11 = 0.0909 or 9% (approx.)

Hence Answer choice (B)
User avatar
EgmatQuantExpert
User avatar
e-GMAT Representative
Joined: 04 Jan 2015
Last visit: 02 Apr 2024
Posts: 3,681
Own Kudos:
Given Kudos: 165
Expert
Expert reply
Posts: 3,681
Kudos: 19,481
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Given

• The cost of fuel increases by 10%.

To Find

• The percent by which the consumption must decrease to keep the overall amount spent the same.


Approach and Working Out

    • Let the cost initially be m and the consumption be 100n.
      o Total amount spent = 100mn
    • Now the cost is 1.1m and consumption is C.’
      o Total amount spent = 1.1mC
    • Hence, 1.1mC = 100mn
      o C =\( \frac{100}{1.1} \)n
      o C = 90.9 n
    • Percent decrease = 100 – 90.9 = 9.09

Correct Answer: Option B
User avatar
Kritisood
Joined: 21 Feb 2017
Last visit: 19 Jul 2023
Posts: 492
Own Kudos:
Given Kudos: 1,090
Location: India
GMAT 1: 700 Q47 V39
Products:
GMAT 1: 700 Q47 V39
Posts: 492
Kudos: 1,220
Kudos
Add Kudos
Bookmarks
Bookmark this Post
if 100 is increased by 25% (ie.,1/4) followed by a decrease of 20% (ie.,1/5) we get 100. In general if any number is increased by a factor of 1/N then it should be decreased by a factor of 1/N+1. This is the relationship between the increase factor and the decrease factor.
Here increase is of a factor of 1/10 hence decrease will be of a factor of 1/11 = 100/11 = approx 9%
User avatar
CrackverbalGMAT
User avatar
Major Poster
Joined: 03 Oct 2013
Last visit: 19 Jul 2025
Posts: 4,847
Own Kudos:
8,652
 [1]
Given Kudos: 225
Affiliations: CrackVerbal
Location: India
Expert
Expert reply
Posts: 4,847
Kudos: 8,652
 [1]
Kudos
Add Kudos
1
Bookmarks
Bookmark this Post
Often, i've seen students struggling with the concept of % increase and %decrease.

The age old % increase or % decrease equation = \(\frac{Final \space - \space Initial}{Initial} * 100\), does sometimes make us think and waste precious seconds.

Simple method to remember:


We are sure is that the Numerator will always be "Difference of values" * 100. It is the denominator that we focus on.


% Increase is an increase which indicates that a value going up. Therefore, the denominator will be a value going in the opposite direction, and hence we choose the smaller of the 2 numbers.

% \(Increase = \frac{Difference}{Smaller \space of \space the \space 2 \space numbers}*100\)


Similarly, % Decrease is an decrease which indicates that a value going down. Therefore, the denominator will be a value going in the opposite direction, and hence we choose the larger of the 2 numbers.

% \(Decrease = \frac{Difference}{Larger \space of \space the \space 2 \space numbers}*100\)

The arrows as shown in the attachment makes it easier to remember.


In the question above:


The question asks by what % should the fuel consumption decrease, so this is a % decrease question.

Let original cost = 100

Therefore the new cost is = 110 (10% Increase)

% \(Decrease = \frac{Difference}{Larger \space of \space the \space 2 \space numbers}*100 = \frac{110 - 100}{110}*100 = \frac{100}{11} = 9.09 \approx 9\)


Option B

Arun Kumar
Attachments

% Increase and %Decrease.jpg
% Increase and %Decrease.jpg [ 600.79 KiB | Viewed 14801 times ]

User avatar
Kinshook
User avatar
Major Poster
Joined: 03 Jun 2019
Last visit: 19 Jul 2025
Posts: 5,703
Own Kudos:
Given Kudos: 161
Location: India
GMAT 1: 690 Q50 V34
WE:Engineering (Transportation)
Products:
GMAT 1: 690 Q50 V34
Posts: 5,703
Kudos: 5,238
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Given: The cost of fuel increases by 10%.
Asked: By what % must the consumption of fuel decrease to keep the overall amount spent on the fuel same?

Amount spent on the fuel = cost of fuel * consumption of fuel
If consumption of fuel is x and cost of fuel if y

Amount spent on the fuel = xy = x'*1.1y
x' = x/1.1 = 10x/11 = .909x = (1-.091)x
Consumption is to reduce by 9% approx

IMO B
User avatar
LeopardLiu
Joined: 23 Aug 2021
Last visit: 05 Dec 2023
Posts: 94
Own Kudos:
Given Kudos: 74
Posts: 94
Kudos: 145
Kudos
Add Kudos
Bookmarks
Bookmark this Post
1.1 Cost*K*Consumption = Cost * Consumption, where K is a constant.

1.1K=1 ==> K=1/1.1 = 0.90909 1-K=0.09 Thus, Answer B
 1   2   
Moderators:
Math Expert
102625 posts
PS Forum Moderator
698 posts