Dakota runs a half-marathon every 6 weeks, and Madison runs a half-marathon every 15 weeks. If they start now, in how many weeks will they again be running a half marathon at the same time?

30 weeks. You continue to find the factors of each number and find one that’s the same.

6, 12, 18, 24, 30
15, 30

Both lines have the 30 in common. So it’ll take 30 weeks until they run a half- marathon together