What are all the common factors of15,45 and90

First find the factors of 15:

1, 3, 5, 15

Next find the factors of 45:

1, 3, 5, 9, 15, 45

Then, find the factors of 90:

1,3,5,6,9,10, 15, 18, 30, 45

Lastly, find the factors that are the same in all of the numbers:

1,3,5, 15

5, 15, 1, 3 are all common factors