Jacob can run a lap around a track in 3 minutes, and Betty can run a lap around the track in 5 minutes. If they both start at the exact same time, how long will it take until they cross the start line again together?

15 minutes until they cross the start line again together

Step-by-step explanation:

Jacob will finish his laps in these following minutes, supposing he starts at 0:

3 minutes, 6 minutes, 9 minutes, 12 minutes, 15 minutes, ...

That is, all instants of time which are multiples of three.

Betty

5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, ...

That is, all instants of time which are multiples of five.

If they both start at the exact same time, how long will it take until they cross the start line again together?

This is the lesser common multiple of 3 and 5, which is 15.

So 15 minutes until they cross the start line again together

15 minutes

Step-by-step explanation:

This question is quite simple if you read it properly and then try to digest it.

It says that Jacob and Betty decided to get a run around the track.

Jacob can run a lap around a track in 3 minutes, and Betty can run a lap around the track in 5 minutes.

If they both start at the exact same time, how long will it take until they cross the start line again together?

Let's do Jacobs calculation first.

If he can run a lap around the track in 3 minutes, then each time he reaches the start line will be:

3,6,9,12,15,18,21,24 ... etc (all are multiples of 3)

And for Betty's calculation, she runs a lap around the track in 5 minutes, them each time he reaches the start line will be 5,10,15,20,25,30 etc.

If you take a look at both their time frames, you'd realize that the only time that both of their times matched was in 15 minutes