LCM means least common multiple(link). It is the smallest number which is in the table of the given numbers.
Find LCM of 4 & 6
LCM of 4 & 6 = 2 × 2 × 3
= 12
Find LCM of 12 & 16
LCM of 12 & 16 = 2 × 2 × 2 × 2 × 3
= 4 × 4 × 3
= 48
Find LCM of 20, 28, 36
LCM of 20, 28, 36 = 2 × 2 × 3 × 3 × 5 × 7
= 4 × 9 × 35
= 1260
Find LCM of 13 & 23
LCM of 13 & 23 = 13 × 23
= 299