155 is not a Prime number, so it can't be a prime factor.
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You need at least two numbers to find a GCF.
Prime factor each number and choose the common factors to multiply.85=5x17155=5x315 is the greatest common factor.
1, 5, 31, 155 5 x 31 = 155
5 x 31 = 1557 x 31 = 217The GCF is 31.
35 can be a factor, but it is not prime.