First write all the numbers as prime factors
24 = 2 x 2 x 2 x 3
36 = 2 x 2 x 3 x 3
40 = 2 x 2 x 2 x 5
We can ignore duplicate factors so the lowest common multiple is 2 x 2 x 2 x 3 x 3 x 5 = 360
You could also list the multiples of each number:
24, 48, 72, 96, 120, 144, 168, 192, 216, 240, 264, 288, 312, 336, 360, ...
36, 72, 108, 144, 180, 216, 252, 288, 324, 360, ...
40, 80, 120, 160, 200, 240, 280, 320, 360, ...
The lowest common multiple is therefore 360