GCM(18, 42) = 6
If GCM stands for Greatest Common Multiple, there is no such number. This is because if x is claimed as the GCM then 2x is a common multiple of both 6 and 8 and 2x > x.
12
2.
The answer is 6
The LCM is 270. The GCM is infinite.
The LCF is 1. The GCF is 18. The LCM is 108. The GCM is infinite.
192.
The greatest common multiple is infinite.
54 is the mudaine answer, but it's 42 in base 13 math (see the Hitchhiker's Guide).
54
The GCM is infinite.