To rewrite the expression as a multiple of a sum of two numbers with no common factor, we're supposed to find the greatest common factor between the terms and factor it out. We can see that both 15 and 24 are divisible by 3. So let's factor the 3 out. Since the 3 goes into 15 five times and into 24 eight times, the expression can be rewritten like this.
15 + 24 = 3(5 + 8)
