To get the Least Common Multiple (LCM) of 20 and 42 we need to factor each value first and then we choose all the factors which appear in any column and multiply them:
20: 2 2 5
42: 2 3 7
LCM: 2 2 3 5 7
The Least Common Multiple (LCM) is: 2 x 2 x 3 x 5 x 7 = 420
Sorry the chart looks messy -_-