The least number of pens required to make complete sets of 20, 24, 30, are?
This is the same as finding a Lowest Common Denominator
20 = 2*2*5
24 = 2*2*2*3
30 = 2*3*5
so we need: 2*2*2*3*5 or 120
20 = 2*2*5
24 = 2*2*2*3
30 = 2*3*5
so we need: 2*2*2*3*5 or 120