Answer:

The factors of a whole number are the whole numbers which divide into it exactly.

The factors of 20 are 1, 2, 4, 5, 10 and 20.
The factors of 30 are 1, 2, 3, 5, 6, 10, 15 and 30.
The common factors of 20 and 30 are 1, 2, 5 and 10.