when you see the would 'of' it generally means to multiply.
what you're going to do is take the one from the fraction [tex]\frac{1}{10} [/tex] 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?