This is a weighted avg question. In weighted averages, the overall is always between the averages of the two groups (male / female). How close the weighted average is to each group is determined by the ratio of one group size to the other. For instance if there are twice as many males as females, the weighted average will be twice as close to the males' datapoint as to the females'
We already know the datapoints for males (72%) and females (80%). To find the fraction of students who are male, we need the ratio of male to female (or male to total). Good rephrases would include "
What is m/(m+f)?" "
What is m/f?" and "
what is the weighted average?"
(1) gives us no way to determine the ratio of male to female
(2) The weighted average (75%) is closer to the males' datapoint (72%) than to the females' (80%). Based on what I wrote in the first paragraph, the weighted average's proximity to each datapoint is determined by the ratio of group sizes, so this statement gives us the means to find the ratio of males/females. It's sufficient.
If you wanted to do the math from (2) you would write "72% of all males and 80% of all females is the same as 75% of all students" --> .72m+.80f=.75(m+f). This can be solved for the ratio m/f.
The answer is
B. If the above doesn't make sense, a step by step video may help: This is
GMATPrep question 1107
-Patrick
