Take the long way out by forming equations when in doubt

A. 3/4 M+1/2F = 300
B. M+F = 500

Simplifying A.
(3M + 2F)/4 = 300 or
3M + 2F = 1200 ...... (C)

Also Multiplying Eqn B by 2 we get
2M+2F = 1000......... (D)

Now solving A and B for values of M or F, we subtract D from C

M= 200 and hence F = 300.

Now just to be sure, recheck.

1/2 of 300 = 150 + 3/4 of 200 = 150 ( both of whih adds up to 300 = Eqn A.

Note: Won't be writing so many steps while calculating for GMAT

Cheers

S

For problems involving overlapping sets,always use the table approach. MGMAT advises this

Attatched solution.