It is true. Let's walk through a few multiples of 9.
9 x 1 = 9 9 x 2 = 18 9 x 3 = 27 9 x 4 = 36 9 x 5 = 45 ... 9 x 115 = 1035 9 x 116 = 1044
As you can see, when you multiply two odd numbers, your product is also negative. A negative number cannot be divisible by 18 because 18 is even. However, all even multiples of 9 WILL be divisible by 18 because when you multiply 9 by the next even number, you are really just adding 18 each time.
