In how many ways 5 boys seat on the 8 chairs?

Answer 1

not enough information:
Are the chairs in a line?
Are the chairs in a circle?

Suppose they are in a line.
Number of ways to choose 5 of the 8 chairs to seat the boys
= C(8,5) = 56

but for each of those choices, we can arrange the 5 boys in 5! ways.

number of ways to seat 5 boys in 8 chairs
= 56(5!) = 6720

This is my interpretation of your question.