Answer:

sum means addition

3/4+4/5

the common denominator is 20

so

3/4= ?/20 20÷4=5 then 3×5=15 3/4= 15/20

4/5= ?/20 20÷5=4 then 4×4=16 4/5= 16/20

15/20+16/20

Answer: 31/20