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So the numbers you have are 20, 15 and 8

The Greatest Common Factor (GCF) is well... is the name itself

We do the prime factorization of the numbers:
20=2^2*5 \\ 15=3*5 \\ 8= 2^{3}

The GCF is 1 since the three don't have a common factor.

The LCM is  computed by getting the greatest power of 2 from the given, greatest power of 3, etc. and multiplying them.

The greatest power of 2 is 3 so 2^3
The greatest power of 3 is 1 so 3
And the greatest power of 5 is 1 so 5


The LCM is 120