Mixture A is 20 percent alcohol, and mixture B is 50 percent alcohol.

HI All,

This question can also be solved by TESTing THE ANSWERS.

We're told that two Mixtures have alcohol contents of 20% and 50% respectively. We're also told that mixing 15 total gallons of the two mixtures gives us a combined mixture that is 30% alcohol content. We're asked how many Gallons of Mixture A (the 20% mixture) are in the 15 Gallon Total.

Since the alcohol content of the final mixture is 30%, and 30% is CLOSER to 20% than it is to 50%, there MUST be MORE of the 20% mixture than the 50% mixture. Since there are 15 total gallons, MORE than 7.5 of those Gallons will be from Mixture A.

With 2 possible Answers (D and E), I'll TEST the easier option first.

Answer D: IF....we have 10 Gallons of the 20% mix, then we have 5 Gallons of the 50% mix....

[10(.2) + 5(.5)]/15 =
[2 + 2.5]/15 =
4.5/15 =
9/30 =
3/10 =
30%

This is an exact MATCH for what we were told, so this MUST be the answer.

Final Answer:

GMAT assassins aren't born, they're made,
Rich