The problem gives you all the numbers you need; 400 and 160. All you need to do for this one is set the problem up as a fraction and then simplify (and then convert to a percentage if you need that).
Since the total number of students is 400 that number is the denominator (on the bottom). We are trying to figure out the probability of who, chosen at random, likes math. This number is a given 160 so it is the numerator (on top).
160/400
Simplify
2/5
or
40%
