Hi All,
I'm a big fan of TESTing VALUES in these types of questions (the approach that Bunuel used). You can also solve this problem by making a table, taking some basic notes and doing a bit of arithmetic. Let's break this prompt into 'pieces':
1) An empty fuel tank is filled with brand Z gasoline.
Tank is FULL of Z
2) When the tank is half empty, it is filled with brand Y gasoline (thus, HALF the Z is gone and the other half of the tank is filled with Y).
(1/2)Z + (1/2)Y
When the tank is half empty again, it is filled with brand Z gasoline (thus, HALF of what was there is gone and the other half is filled with Z).
(1/4)Z + (1/4)Y + (1/2)Z
When the tank is half empty again, it is filled with brand Y gasoline (thus, HALF of what was there is gone and the other half is filled with Y.
(1/8)Z + (1/8)Y + (1/4)Z + (1/2)Y
At this time, what percent of the gasoline in the tank is brand Z?
We now have (1/8 + 1/4)Z and (1/8 + 1/2)Y
1/8 + 1/4 = 3/8 so we have 3/8 = 37.5% Z
Final Answer:
GMAT assassins aren't born, they're made,
Rich
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