To find the GCF of 36, 42, and 54, we must find the prime factors of all of the 3 numbers.
36=12*3=4*3*3
42=6*7=2*3*7
54=6*9=2*3*3*3
For the GCF, you must include any number that is a factor of EVERY number in the list, and you write it the least amount of times that it shows up. Only 2 and 3 are factors of all three of the numbers you listed. The least 2 shows up is 1 time, and the least 3 shows up is one time.
So the GCF is 2*3=6
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