Denominators: 75 and 45
Factor each denominator into its primes:
75 = 3 * 5 * 5
45 = 3 * 3 * 5
The least common denominator is made by multiplying the list of primes needed to make both denominators.
What?
If we have 3 * 3 * 5 * 5, that's enough to make 75 (3 * 5 * 5) and to make 45 (3 * 3 * 5), so the common denominator is 3 * 3 * 5 * 5 = 225.
Answer: 225