Lowest Common Multiple of 70 and 40?

Answer 1

280

There are several ways to do this problem.

The most practical way is to make a list of the multiples of 70:

70, 140, 210, 280, 350, 420, etc.

Then make a list of the multiples of 40:

40, 80, 120, 160, 200, 240, 280, 320

What is the lowest number that appears in both lists? 280.

That is not the method in any textbook!

The textbook method is to factor out any common factors from one of the numbers. The factors of 70 are 2, 5, 7 (i.e. 2x5x7 = 70). The factors of 40 are 2, 2, 2, 5. Both numbers have a 2 and a 5 as factors. Take out a 2 and a five from the second list, then multiply all the factors that are left times the first number. 2x2x70 = 280.

There are other related ways to do LCM problems. Different textbooks, grades, states, and countries emphasize slightly different methods.