What is the highest common factor of 55?

Answer 1

A single number cannot have a "common" factor, though it has factors: 1, 5,11.

If you ask for the factors of 40 and 50, then for 40 you have 1, 2, 4, 5, 8, 10, 20, 40, while for 50 you have 1, 2, 5, 10, 25 and 50.

In both of those sets you van find 1, 2, 5,10. Because they are in both sets they are "common" to both 40 and 50.

The highest of the common ones is 10, so this is the "highest common factor" of 40 and 50.

No higher factor will divide into both 40 and 50.