We know that 3/5 of the answer is right and 8 are wrong. If we do 5/5-3/5, we get 2/5 which is the amount of problems that are wrong or in other 8 wrong.
3/5=?
2/5= 8 wrong
If we make 2/5 into 1/5 we get 4 wrong answer.
Then we multiply 1/5 and 4 by 5 we get: 5/5 and 20 problems.
So there is 20 problems on the test.