Chapter 1 - Some Basic Concepts of Arithmetic and Algebra - 1.2 - Prime and Composite Numbers - Problem Set 1.2 - Page 13: 85

90

Multiples of $9$ are ${9,18,27,36,45,54,63,72,81,90,99....}$ Multiples of $15$ are ${15,30,45,60,75,90,105...}$ Multiples of $18$ are ${18,36,54,72,90...}$ Therefore, the lowest common multiple amongst these three numbers is $90$.