A
------ W
.
..
...
....
.....
......
.......
T__TMG
I'm not sure if this will work when its formatted but it should explain in simple terms.
The wind strength is W to A and your intended course is A to T. To counter the drift, you must fly the distance of A to TMG and as this distance is longer than A-T, your ground speed will (A to TMG)/IAS not (A to T)/IAS.
Not a dig at you (and I am a big fan of GPS), but Nav exams should be computer free.