What is the greatest common factor of 51 and 85?

The greatest number which divides both 51 and 85 is 17. So, greatest common factor is 17.

Alternate method:

P.F. of 51 = 3x17

P.F. of 85 = 5x17

Clearly, from the P.F. of both numbers 17 is the greatest common factor.
The GCF is 17.
51 and 85's highest common factor is 17.