To find LCM, multiply two number by 1, 2, 3 ..., find the least number that can multiply by both. If you can't find the LCM when you multiply the numbers, just keep multiply until you get LCM
18: 18, 36, 54, 72, 90,......
24: 24, 48, 72, 96, 120,...
We see that 18*4 = 72 and 24*3= 72, 72 is the smallest number that can multiply by both so D. 72 is the LCM of 18 and 24
