The answer is b.
15/20. multiply 3/4 by 5 on both the top and bottom,, the result is 15/20. Once you have 15/20 you take the other fraction (2/5) and multiply both the top and bottom by 4. You should end up with 8/20 now that both fractions have the same denominator you can subtract them. Once you do so the denominator will stay the same except the numerator you end up with 7/20 with this fraction you can add 8/20 when they are added together you should get 15/20.