Ratio/percent questions occur frequently on the GMAT; one of the great things about these questions is that they can be solved in a variety of ways:
Here, we have a ratio of Apples:Peaches:Grapes in the ratio of 6:5:2. We're told that 39 pounds of this mixture is prepared and we're asked how many MORE pounds of Apples are there than pounds of Grapes.
Since the answer choices are numbers, we can use them to our advantage and avoid some of the math. From the original ratio, we know that....
The pounds of....
Apples will be a multiple of 6
Peaches will be a multiple of 5
Grapes will be a multiple of 2
Total will be a multiple of 13
So, the difference in the pounds of Apples and Grapes will be 6 - 2 = some multiple of 4 pounds.
If we had 13 pounds total, then the difference would be 4 pounds. Since the total is MORE than 13 pounds, the difference will be MORE than 4 pounds (but still a multiple of 4). Only one answer fits that pattern:
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