Find the l.c.m. of 15 and 18

To calculate the least common multiple of 15 and 18, we'll
factor 15 and 18 into their prime factors.

15 =
3*5

18 = 2*3*3

Now, we'll
consider the different factors from both numbers and we'll multiply
them.

We notice that we have 3 and 3^2 as factors in 15 and
18. We'll choose the factor that has the highest exponent. In this case is
3^2.

lcm [15,18] = 2 * 3^2 * 5 =
90

Another method would be to write several
integers divisible by 15 and several integers divisible by
18.

D15 =
15,30,45,60,75,90,105,...

D18
=
18,36,54,72,90,108,...

We
notice that the first positive integer divisible by both 15 and 18 is
90.