Let's start with the multiples of each number.
35, 70, 105, 140, 175, 210, 245, 280, 315, 350, 385, 420, 455 . . .
12, 24, 36, 48, 60, 72, 84, 96, 108, 120, 132, 144, 156, 168, 180, 192, 204, 216, 228, 240, 252, 264, 276, 288, 300, 312, 324, 336, 348, 360, 372, 384, 396, 408, 420, 432 . . .
20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300, 320, 340, 360, 380, 400, 420, 440 . . .
What is the smallest number in those three lists?
Gee -- but that was a lot of work. I would have saved a lot of time if I'd multiplied 35 times 12 = 420. Since it is also divisible by 20 -- that must be the answer.
http://www.helpwithfractions.com/least-common-multiple.html
LCM OF 35,12,20
2 answers
420 because
35=5 x 7
12=3 x 2 x 2
20=2 x 2 x 5
= 5 x 7 x 3 x 2 x 2
35=5 x 7
12=3 x 2 x 2
20=2 x 2 x 5
= 5 x 7 x 3 x 2 x 2