What you need to do is to find a multiple of 20 and 16 in other words a number that can multiply into 20 and 16. You have 2 and you have 4. So now this is how it looks like.
When adding fractions, you want the denominators to be the same number, so you have to multiply 31/2 by four for the numerator and denominator by 4 to have the denominators to both be 8. So the fraction on the left will be 124/8 + 1/8 =129/8. That fraction is in its simplest form.
