November 20, 2018 November 20, 2018 09:00 AM PST 10:00 AM PST The reward for signing up with the registration form and attending the chat is: 6 free examPAL quizzes to practice your new skills after the chat. November 20, 2018 November 20, 2018 06:00 PM EST 07:00 PM EST What people who reach the high 700's do differently? We're going to share insights, tips and strategies from data we collected on over 50,000 students who used examPAL.
Author 
Message 
TAGS:

Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 50661

The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
07 Jul 2017, 01:10
Question Stats:
59% (01:22) correct 41% (01:22) wrong based on 210 sessions
HideShow timer Statistics




Senior PS Moderator
Joined: 26 Feb 2016
Posts: 3307
Location: India
GPA: 3.12

The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
07 Jul 2017, 01:54
One of the methods of solving this question is by substituting values. Assume the cost of the fuel to be 10$ and the consumption of fuels to be 10 units. Now, the overall amount spent is 100$ Consider the cost of fuel to increase by 10%, and fuel becomes 11$. Hence to spend the same amount on fuel we would have to spend x units of fuel. \(11x = 100\) >\(x = \frac{100}{11} = 9.09\), which is approximately 9.1$ Hence there is a decrease is of 9%(Option B), since consumption goes down from 10$.
_________________
You've got what it takes, but it will take everything you've got




CEO
Status: GMATINSIGHT Tutor
Joined: 08 Jul 2010
Posts: 2701
Location: India
GMAT: INSIGHT
WE: Education (Education)

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
07 Jul 2017, 03:44
Bunuel wrote: 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
_________________
Prosper!!! GMATinsight Bhoopendra Singh and Dr.Sushma Jha email: info@GMATinsight.com I Call us : +919999687183 / 9891333772 Online OneonOne Skype based classes and Classroom Coaching in South and West Delhi http://www.GMATinsight.com/testimonials.html
ACCESS FREE GMAT TESTS HERE:22 ONLINE FREE (FULL LENGTH) GMAT CAT (PRACTICE TESTS) LINK COLLECTION



Senior SC Moderator
Joined: 22 May 2016
Posts: 2111

The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
07 Jul 2017, 05:49
Bunuel wrote: 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% Amount of consumption * cost of fuel = total overall amount spent on fuel. To keep the original overall amount spent the same when fuel cost increases, the multiplication factor for the cost increase will be the inverse of the multiplication factor needed to achieve the desired decrease in consumption. 10% increase = 1\(\frac{1}{10}\) =\(\frac{11}{10}\) Flip that fraction: \(\frac{10}{11}\) is the multiplier for the percent by which consumption must decrease from the nowincreased original. There are two traps in this question. The first informs the second when calculating. Trap one, wellknown: if you increase by a percent, then decrease by that same percent, you do not get to the same place you started. The consumption decrease is not 10%. Easy enough, but that trap feeds into the second trap because of the way the arithmetic falls out here, whether you use actual quantities (as above) or multipliers. Decrease needed to offset nowincreased original = \(\frac{10}{11}\) = .909090... Remember the first trap, and round up to .91 to avoid .10 as the answer (1  .90 = .10, which is wrong). .91 is a decrease of 9%. Answer B.



Director
Joined: 21 Mar 2016
Posts: 522

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
09 Jul 2017, 07:44
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



Verbal Forum Moderator
Status: Greatness begins beyond your comfort zone
Joined: 08 Dec 2013
Posts: 2125
Location: India
Concentration: General Management, Strategy
GPA: 3.2
WE: Information Technology (Consulting)

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
09 Jul 2017, 19:06
Bunuel wrote: 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
_________________
When everything seems to be going against you, remember that the airplane takes off against the wind, not with it.  Henry Ford The Moment You Think About Giving Up, Think Of The Reason Why You Held On So Long +1 Kudos if you find this post helpful



DS Forum Moderator
Joined: 21 Aug 2013
Posts: 1370
Location: India

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
09 Jul 2017, 20:44
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



Target Test Prep Representative
Status: Head GMAT Instructor
Affiliations: Target Test Prep
Joined: 04 Mar 2011
Posts: 2830

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
12 Jul 2017, 15:31
Bunuel wrote: 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(1n/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
_________________
Jeffery Miller
Head of GMAT Instruction
GMAT Quant SelfStudy Course
500+ lessons 3000+ practice problems 800+ HD solutions



Intern
Joined: 03 Jan 2017
Posts: 12
Location: India
Concentration: Finance, Economics

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
14 Jul 2017, 16:55
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



Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 8560
Location: Pune, India

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
07 Sep 2017, 23:14
Bunuel wrote: 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)
_________________
Karishma Veritas Prep GMAT Instructor
Learn more about how Veritas Prep can help you achieve a great GMAT score by checking out their GMAT Prep Options >
GMAT selfstudy has never been more personalized or more fun. Try ORION Free!



Senior Manager
Status: love the club...
Joined: 24 Mar 2015
Posts: 267

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
09 Sep 2017, 04:38
VeritasPrepKarishma wrote: Bunuel wrote: 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) thanks mam you are sooooo great ... Since, 10/11 implies 1  1/11, we can safely assume 1= "previous number of units" which should be cut by 1/11 which implies 9.09% ... thanks mam ..



Manager
Status: Math Tutor
Joined: 12 Aug 2017
Posts: 73
WE: Education (Education)

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
09 Sep 2017, 09:57
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 \(\frac{1}{41}\) = \(\frac{1}{3}\) = 33.33% to get the original value.
_________________
Abhishek Parikh Math Tutor Whatsapp +919983944321 Mobile +971568653827 Website: http://www.holamaven.com



Senior Manager
Status: love the club...
Joined: 24 Mar 2015
Posts: 267

The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
09 Sep 2017, 10:36
HolaMaven wrote: 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 \(\frac{1}{41}\) = \(\frac{1}{3}\) = 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)



Manager
Status: Math Tutor
Joined: 12 Aug 2017
Posts: 73
WE: Education (Education)

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
10 Sep 2017, 00:56
gmatcracker2017 wrote: HolaMaven wrote: 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 \(\frac{1}{41}\) = \(\frac{1}{3}\) = 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.
_________________
Abhishek Parikh Math Tutor Whatsapp +919983944321 Mobile +971568653827 Website: http://www.holamaven.com



Manager
Status: Math Tutor
Joined: 12 Aug 2017
Posts: 73
WE: Education (Education)

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
10 Sep 2017, 08:33
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% = \(\frac{1}{100}\)
_________________
Abhishek Parikh Math Tutor Whatsapp +919983944321 Mobile +971568653827 Website: http://www.holamaven.com



NonHuman User
Joined: 09 Sep 2013
Posts: 8822

Re: The cost of fuel increases by 10%. By what % must the consumption of
[#permalink]
Show Tags
19 Sep 2018, 08:28
Hello from the GMAT Club BumpBot! Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up  doing my job. I think you may find it valuable (esp those replies with Kudos). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
GMAT Books  GMAT Club Tests  Best Prices on GMAT Courses  GMAT Mobile App  Math Resources  Verbal Resources




Re: The cost of fuel increases by 10%. By what % must the consumption of &nbs
[#permalink]
19 Sep 2018, 08:28






