Which of the following are factor pairs of 54 ? Select all that apply. 2x108, 2x27, 4x18,6x9, 4x21

The factor pairs of 54 are:

2 x 27
3 x 18
6 x 9

So, 2 x 27, 4 x 18, and 6 x 9 are the correct factor pairs of 54.