#### Answer

20

#### Work Step by Step

The greatest common factor is the greatest number that can be multiplied by a whole number to equal 40, 80, and 100. 20 can be multiplied by 2 to get 40, by 4 to get 80, and by 5 to get 100. There is no number above 20 that can be multiplied by a whole number to get 40, 80, and 100.