Is 80 the greatest common factor of 24 and 42?

No; that would be 6.

The factors of 24 are 1, 2, 3, 4, 6, 8, 12, and 24.

The factors of 42 are 1, 2, 3, 6, 7, 14, 21, and 42.

The common factors are 1, 2, 3, and 6, so the greatest common factor is 6.