The first step in determining lowest common factor, (LCM) is to express the numbers as factors.
For the first option,
3 = 3 * 1
6 = 3 * 2 * 1
9 = 3 * 3 * 1
The highest power of 3 is 2. the highest power of 2 is 1. thus, the LCM is
3^2 * 2^1 = 9 * 2 = 18
For the second option,
12 = 2 * 2 * 3
15 = 3 * 5
The highest power of 2 is 2. The highest power of 3 is 1. The highest power of 5 is 1. thus
LCM = 2^2 * 3^1 * 5^1 = 4 * 3 * 5 = 60
For the last option,
4 = 2 * 2
8 = 2 * 2 * 2
The highest power of 2 is 3. Thus,
LCM = 2^3 = 8
Thus, the incorrect math sentence is the third sentence