Can you list the factors of each of those numbers (the numbers that divide evenly into them)? Of those three lists, what's the highest number present in all three?
The greatest common factor for 16, 27, and 20 is 1. The factors for 16 are 1,2,4,8,16. The factors for 27 are 1,3,9,27. The factors for 20 are 1,2,4,5,10,20.
