Last visit was: 14 Jul 2026, 16:01 It is currently 14 Jul 2026, 16:01
Home
Messages
Tests
Schools
Events
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.
Go to My Error Log Learn more
Close
Request Expert Reply
Confirm Cancel
Back to Forum
Create Topic
   1   2 
avatar
abhinavsareen1
avatar
Joined: 22 Dec 2016
Last visit: 04 Sep 2017
Posts: 4
Own Kudos:
2
 [2]
Given Kudos: 10
Posts: 4
Kudos: 2
 [2]
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 21 Jul 2017, 21:05
Kudos
Add Kudos
2
Bookmarks
Bookmark this Post
Besides some easy methods we can also do this question with the help of allegation approach:-
Initially, there was 5% ethanol and we have to add pure ethanol i.e 100% ethanol and the final concentration we need to obtain is 10%.
5%-------------------------100%
------------10%----------
90%-------------------------5% (Use allegation method)

so we have final ratio 90:5 or 18:1
We know we have 5% ethanol 20ml so the amount of 100% ethanol required assume y is:- [18/20 = 1/y] => y = 10/9 (Simple ratios)
User avatar
BrentGMATPrepNow
User avatar
Major Poster
Joined: 12 Sep 2015
Last visit: 31 Oct 2025
Posts: 6,733
Own Kudos:
36,917
 [1]
Given Kudos: 799
Location: Canada
Expert
Expert reply
Posts: 6,733
Kudos: 36,917
 [1]
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 28 Apr 2018, 08:13
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
above720
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?

A. 9/10
B. 1
C. 10/9
D. 20/19
E. 2
Show more

Bob just filled his car's tank with 20 gallons of gasohol, a mixture containing of 5% ethanol and 95% gasoline.
5% of 20 gallons is 1 gallon.
So, PRESENTLY, there is 1 gallon of ethanol in the car's tank.


We want to add some PURE ethanol to the tank in order to get a 10% mixture of ethanol.
Let x = number of gallons of pure ethanol we ADD to the tank.

FACT #1: Once we add x more gallons of pure ethanol, the car's tank contains 1+x gallons of ethanol.

FACT #2: Once we add x gallons of ethanol, the car's tank contains a TOTAL of 20+x gallons of mixture.

We want the tank to have a 10% mixture of ethanol. In other words, we want the mixture in the tank to contain 1/10 ethanol.
So, we can write the equation: (1+x)/(20+x) = 1/10
Cross multiply to get: 10(1 + x) = 1(20 + x)
Expand: 10 + 10x = 20 + x
Rearrange: 9x = 10
Divide both sides by 9 to get: x = 10/9

Answer: C

Cheers,
Brent
User avatar
Rajeet123
User avatar
Joined: 15 Aug 2017
Last visit: 06 Apr 2024
Posts: 39
Own Kudos:
11
Given Kudos: 276
Location: India
Schools: HBS '22
Schools: HBS '22
Posts: 39
Kudos: 11
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 28 Jul 2019, 21:55
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Bunuel
gmatpapa
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?
a) 9/10
b) 1
c) 10/9
d) 20/19
e) 2

Is it that we dont't need to have 20 litres of gasohol? Because if we do, I think the answer will be B.

Currently there is 5% of ethanol out of 20 litres of fuel that means the mixture contains 1 litre of ethanol. To have 10% ethanol, Bob needs to have 2 litres of ethanol, therefore he needs to add 1 more litre of ethanol.

Please read the question carefully: "how many gallons of ethanol must he add ..." add not replace.

19 gallons of gasoline which is now in the tank must comprise 90% of the whole fuel after adding some amount of ethanol --> thus whole amount of fuel after adding must be 19/(9/10)=190/9 gallons --> amount of the ethanol which Bob must add is 190/9-20=10/9 gallons.

Answer: C.
Show more

We already have 19L of gasoline and the car runs best at 10% ethanol. So we need 10% of 19 ie 1.9L of ethanol. And since we already have 1L of ethanol, we need an additional of 1.9 - 1 = 0.9L of ethanol. Hence A. Can you please tell me where am I going wrong?
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 14 Jul 2026
Posts: 111,929
Own Kudos:
827,454
 [2]
Given Kudos: 109,329
Products:
GMAT Club Tests
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 111,929
Kudos: 827,454
 [2]
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 28 Jul 2019, 22:09
Kudos
Add Kudos
1
Bookmarks
Bookmark this Post
Rajeet123
Bunuel
gmatpapa
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?
a) 9/10
b) 1
c) 10/9
d) 20/19
e) 2

Is it that we dont't need to have 20 litres of gasohol? Because if we do, I think the answer will be B.

Currently there is 5% of ethanol out of 20 litres of fuel that means the mixture contains 1 litre of ethanol. To have 10% ethanol, Bob needs to have 2 litres of ethanol, therefore he needs to add 1 more litre of ethanol.

Please read the question carefully: "how many gallons of ethanol must he add ..." add not replace.

19 gallons of gasoline which is now in the tank must comprise 90% of the whole fuel after adding some amount of ethanol --> thus whole amount of fuel after adding must be 19/(9/10)=190/9 gallons --> amount of the ethanol which Bob must add is 190/9-20=10/9 gallons.

Answer: C.

We already have 19L of gasoline and the car runs best at 10% ethanol. So we need 10% of 19 ie 1.9L of ethanol. And since we already have 1L of ethanol, we need an additional of 1.9 - 1 = 0.9L of ethanol. Hence A. Can you please tell me where am I going wrong?
Show more

The mixture should consist of 10% ethanol and 90% gasoline. So, ethanol should be 10% of the whole mixture NOT 10% of the amount of gasoline as you did.
User avatar
KarishmaB
User avatar
Joined: 16 Oct 2010
Last visit: 14 Jul 2026
Posts: 16,562
Own Kudos:
80,333
 [2]
Given Kudos: 486
Location: Pune, India
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 16,562
Kudos: 80,333
 [2]
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 12 Apr 2022, 23:52
2
Kudos
Add Kudos
Bookmarks
Bookmark this Post
above720
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?

A. 9/10
B. 1
C. 10/9
D. 20/19
E. 2
Show more

Amount of gasoline stays the same. So using gasoline, Ci*Vi = Cf * Vf

95% of 20 gallons = 90% of X gallons

\(\frac{19}{18} * 20 = X\)

X = 190/9

The volume changed from 20 gallons to 190/9 gallons i.e. an addition of 190/9 - 20 = 10/9 gallons was done. So 10/9 gallons of ethanol was added.

Answer (C)

RajatGMAT777
User avatar
Schachfreizeit
User avatar
Joined: 17 Nov 2022
Last visit: 02 Feb 2023
Posts: 110
Own Kudos:
9
Given Kudos: 8
Posts: 110
Kudos: 9
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 20 Dec 2022, 04:44
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Bunuel
gmatpapa
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?
a) 9/10
b) 1
c) 10/9
d) 20/19
e) 2

Is it that we dont't need to have 20 litres of gasohol? Because if we do, I think the answer will be B.

Currently there is 5% of ethanol out of 20 litres of fuel that means the mixture contains 1 litre of ethanol. To have 10% ethanol, Bob needs to have 2 litres of ethanol, therefore he needs to add 1 more litre of ethanol.

Please read the question carefully: "how many gallons of ethanol must he add ..." add not replace.

19 gallons of gasoline which is now in the tank must comprise 90% of the whole fuel after adding some amount of ethanol --> thus whole amount of fuel after adding must be 19/(9/10)=190/9 gallons --> amount of the ethanol which Bob must add is 190/9-20=10/9 gallons.

Answer: C.
Show more


19/(9/10)=190/9 gallons

can you explain the calculation?
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 14 Jul 2026
Posts: 111,929
Own Kudos:
827,454
Given Kudos: 109,329
Products:
GMAT Club Tests
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 111,929
Kudos: 827,454
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 20 Dec 2022, 04:59
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Schachfreizeit
Bunuel
gmatpapa
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?
a) 9/10
b) 1
c) 10/9
d) 20/19
e) 2

Is it that we dont't need to have 20 litres of gasohol? Because if we do, I think the answer will be B.

Currently there is 5% of ethanol out of 20 litres of fuel that means the mixture contains 1 litre of ethanol. To have 10% ethanol, Bob needs to have 2 litres of ethanol, therefore he needs to add 1 more litre of ethanol.

Please read the question carefully: "how many gallons of ethanol must he add ..." add not replace.

19 gallons of gasoline which is now in the tank must comprise 90% of the whole fuel after adding some amount of ethanol --> thus whole amount of fuel after adding must be 19/(9/10)=190/9 gallons --> amount of the ethanol which Bob must add is 190/9-20=10/9 gallons.

Answer: C.


19/(9/10)=190/9 gallons

can you explain the calculation?
Show more

Sure:

\(\frac{19}{(\frac{9}{10})}=19*\frac{10}{9}=\frac{190}{9}\).
User avatar
MonishBhawale
User avatar
Joined: 15 Jun 2021
Last visit: 01 May 2025
Posts: 32
Own Kudos:
36
Given Kudos: 270
Posts: 32
Kudos: 36
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 06 Apr 2023, 23:26
Kudos
Add Kudos
Bookmarks
Bookmark this Post
above720
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?

A. 9/10
B. 1
C. 10/9
D. 20/19
E. 2
Show more

by punching options
20=1+19
check the options a and c are inverted so likely answer either of the two
lets check c
1+10/9 +19
19/9+19
2.1+19
21.1 so 10% of this is 2.1 so c is right
User avatar
hemanthPA
User avatar
Joined: 09 Mar 2023
Last visit: 18 Mar 2025
Posts: 47
Own Kudos:
42
Given Kudos: 11
Posts: 47
Kudos: 42
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 23 Nov 2024, 10:28
Kudos
Add Kudos
Bookmarks
Bookmark this Post
above720
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?

A. 9/10
B. 1
C. 10/9
D. 20/19
E. 2
Show more
Buongiorno signore e signori
20 x 5/100 = 1

in 20 gallons of gasohol there's 1 gallon of ethanol (which is 5%), now we gotta figure out how much of ethanol to be added to make the mixture 10% ethanol

1 + x = 0.1(20 + x)
1+x = 2+0.1x
0.9x = 1
x = 1/0.9 = 10/9

Voila!!
User avatar
BraydinWalters
User avatar
Joined: 07 Nov 2024
Last visit: 29 Jun 2026
Posts: 29
Own Kudos:
26
Given Kudos: 81
Location: United States
Posts: 29
Kudos: 26
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 24 Nov 2024, 13:03
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Let's use the Alligation Method:

Greater AmountDesired Amount - Desired Amount
Desired Amount
Lesser AmountGreater Amount - Desired Amount

Now let's bring in the the variables provided:

100% Ethanol Added10 - 5 = 5
10% Ethanol Desired
5% Ethanol Concentration100 - 10 = 90

We now have our ratio of 90:5 or 18:1.

Review:
  • We have a solution with 5% ethanol
  • We're trying to raise this concentration to 10% ethanol
  • To raise the ethanol concentration, we're adding a 100% ethanol solution
  • We are starting with 20 gallons of liquid

20 gallons (current solution) = 18/1 x Ethanol

20 = ethanol
18

10/9

C
User avatar
Adit_
User avatar
Joined: 04 Jun 2024
Last visit: 13 Jul 2026
Posts: 877
Own Kudos:
310
Given Kudos: 162
Posts: 877
Kudos: 310
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 17 Mar 2026, 21:48
Kudos
Add Kudos
Bookmarks
Bookmark this Post
1 method is taking gasoline as the constant since it doesnt change and frame it like:
19/20+x = 9/10
x = 10/9

Another method:
Using alligation:

5 100
\ /
10
/ \
90 5

90:5 is the ratio for number of parts of existing vs newly added
90:5 = 18:1

1:1/18 parts.
which means 1 part of 20 gallons is added with 1/18th of total to achieve the required result.

= 20*1/18 = 10/9

above720
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?

A. 9/10
B. 1
C. 10/9
D. 20/19
E. 2
Show more
User avatar
architkap
User avatar
Joined: 07 Apr 2026
Last visit: 14 Jul 2026
Posts: 21
Own Kudos:
35
Given Kudos: 11
Posts: 21
Kudos: 35
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 23 Jun 2026, 03:47
Kudos
Add Kudos
Bookmarks
Bookmark this Post
an alternate approach using options -

I tested picking the option that stood out the most, 10/9 = 1.1

If there are 20 gallons total, and I add 1.1 gallons of ethanol, total is now 21.1 gallons. Initially there was 1 gallon of ethanol in the mixture, and now I added another 1.1 so total 2.1 gallons of ethanol. 2.1 is 10% of 21.1 so valid.

Can anyone tell me if there is any flaw in this method?
User avatar
Adit_
User avatar
Joined: 04 Jun 2024
Last visit: 13 Jul 2026
Posts: 877
Own Kudos:
310
Given Kudos: 162
Posts: 877
Kudos: 310
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 23 Jun 2026, 05:12
Kudos
Add Kudos
Bookmarks
Bookmark this Post
I don't see any flaw there, that is a perfectly sound approach.
architkap
an alternate approach using options -

I tested picking the option that stood out the most, 10/9 = 1.1

If there are 20 gallons total, and I add 1.1 gallons of ethanol, total is now 21.1 gallons. Initially there was 1 gallon of ethanol in the mixture, and now I added another 1.1 so total 2.1 gallons of ethanol. 2.1 is 10% of 21.1 so valid.

Can anyone tell me if there is any flaw in this method?
Show more
User avatar
thalbritter
User avatar
Joined: 30 Jun 2026
Last visit: 14 Jul 2026
Posts: 8
Own Kudos:
1
Given Kudos: 1
Products:
Forum Quiz
Posts: 8
Kudos: 1
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 08 Jul 2026, 01:30
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Target ratio: (1+x)/(20+x) = 1/10

Solve for x:
1+x = 2+0,1x
0,9x = 1
x = 10/9
   1   2 
Moderator:
Bunuel
Math Expert
111929 posts