when you see the would 'of' it generally means to multiply.
what you're going to do is take the one from the fraction

and multiply 90 by it, which would equal 90. Then you take the 10 (the denominator) and divide 90 by it. So it will 1 × 90 and then 90 ÷ 10.
So, you multiply by the numerator and then divide that answer by the denominator. Do you understand?