Hello!
Given the integer in form of fraction, we should know the denominator. We know the denominator n is 3^x * 5^y.
We should know the greatest divisor of 90.
You have to prime factorize 90.
90|2
45|3
15|3
5|5
1
So 90 = 2 × 3^2 × 5^1
We take the greatest numbers with highest exponents.
That means 3^2 × 5 = 45 <- answer