Find the least common multiple (LCM) for each of the following groups of numbers

Question

12, 20, and 35

Answer 1

That would be the number that contains the prime factors of all of the three numbers, for the maximum number of times that they appear in any single number.
12 = 3*2*2
20 = 2*2*5
35 = 5*7

The LCM is 2*2*3*5*7= 420