First, we list the prime factors of all the numbers:
40 = 2 * 2 * 2 * 5
70 = 2 * 5 * 7
80 = 2 * 2 * 2 * 2 * 5
Now, we see the unique factors. They are:
2, 5, and 7
We take each of these the highest number of time it occurs in any of the numbers.
2 occurs 4 times in 80
5 occurs 1 times highest, so does 7
Hence, the LCM would be:
LCM = 2 * 2 * 2 * 2 * 5 * 7
Which is
560
