Can anyone explain in full how to do those kind of questions ?

When dealing with fractions, take the whole as 1. 
in this case, the whole class is 1;
fraction of votes for Andrew: 1-1/2 -1/4 - 1/5
these fractions have different denominators (bottom), to continue, you have to make the denominators the same; 
to do that, I usually count by the largest one. In this case, it is a 5. 5 is not divisible by 2 or 4, so I move on to 10. 10 is still not divisible by 2 and 4, so try 15, then 20.
20 works, so make all the denominators to 20. Remember 1 is 20/20:
20/20- 10/20 -5/20 -4/20 =1/20
so the fraction of votes Andrew got was 1/20.

For answer b, the whole is no longer 1. the whole is now 100.
1/2 of 100=50 for Amy
1/4 of 100=25 for Sophie
1/5 of 100=20 for Ryan
1/20 of 100=5 for Andrew

Notice the numbers add up to be exactly 100

Cwill be your correct answer