how do i solve question of exact multiple

I am slightly doubtful regarding your question but if I have got it right then probably you are asking about the question of this sort.
How many numbers between 100 and 500 are exactly divisible by 5?
Please re-post with an example if your doubt is different.
For this question, the method is (N2 -N1)/5 + 1. This is the case when both the boundary numbers are included. If only one of them is included then it is just (N2 -N1)/5. If none of the boundary numbers are included then it is (N2 -N1)/5 - 1.
Hope that helps.