Mettalicafan, you are correct,common sense should tell the answer at first glance. But as you want the mathematical answer , here it is-

1. Option A alone is not sufficient because it didnt even mention the employment term.

2. suppose there are X student of 25 years age or older, and Y student younger than 25 years.
so

Total employed student=35% of X+ 20% of Y= 0.35X+0.2Y..........................(1)

total student= X+Y
question asks if employed students>= 20% of (X+Y)

from equation (1)

if
0.35X+0.2Y>=0.2(X+Y)
0.35X>=0.2X
0.35>=0.2

which is always right. So B is sufficient.