60, 120, 180 and so on.
20
-12
(6 + √20) ≈ 10.472 and (6 - √20) ≈ 1.528
2 x 10 = 20 2 + 10 = 12
12
12, 24, 36
20
4 and 24 8 and 20 12 and 16
140, 280, 420, 560, 700
The greatest common factor of 12 and 20 is 4.
The Least Common Multiple (LCM) for 12 20 is 60.
A common denominator, though not the least one, is 12*20 = 240.