#### Answer

315

#### Work Step by Step

We find the least common denominator (LCD) (the smallest positive integer that has all the denominators as divisors). First we factor each denominator into its prime factors. Then we find the number of times each prime factor appears most in any denominator. The LCD is the product of these prime factors.
9=3*3
15=3*5
21=3*7
So the LCD must be: 3*3*5*7=315