There are 8 students one the minibus. what fraction of the students are boys

if there's eight students on the minibus then there's 4 boys and 4 girls on the minibus

Well, you can't determine that with this information.
We would need to know the number of girls, or boys.

Like this:

There are 8 students of the mini bus. There are 3 girls. What fraction of the students are boys?

So, there are 8 persons. So the denominator is 8.

?/8

And there is 3 girls. So you would subtract 3 from 8.

Which is 5.

So there is 5 boys!

And the fraction is 5/8 and can not be simplified.