10 = 2 * 5
15 = 3 * 5
It's like you're making a list of parts that you can make either number from.
In this case we want 2 * 3 * 5 because that's the list of parts that we need to make either 10 or 15.
2 * 3 * 5 = 30
Answer: The common denominator is 30.