Common factors of 20 are; 1,2,4,5, and 20. 1x20=20, 2x10=20, 4x5=20, 5x4=20, 20x1+=20. If you have to show your work then just check mine. The factors of 20 are 1,2,4,5,10 and 20. For them to be common, they have to be compared to another set of factors.