Question

A student did 3/5 of his homework and 8 more problems. then his father came and did one fifth of the homework and the remaining 7 problems. how many problems did they solve?

Answer 1

Suppose the total  number of problems be x
this will be given by:
(1/5)x+7+(3/5)x+8=x
Putting like terms together we get:
(1/5x+3/5x)+(7+8)=x
4/5x+15=x
4/5x-x=-15
-1/5x=15
thus
x=75 problems

Answer 2

If x = total number of problems for the homework,

Student:
Number of problems done = (3/5)x + 8

Father:
Number of problems done = (1/5)x + 7

But total number of problems done is x. Then;
x = (3/5)x + 8 + (1/5)x + 7 = (4/5)x + 15
(1-4/5)x = 15
(1/5)x = 15
x = 15*5 = 75.
∴
There are 75 problems.