One bus leaves a stop every 9 minutes a second bus leaves every 15 minutes if they both leave at 1:25 at what time will they next leave together show work

9 = 3^

15 = 3 x 5

both of them have 3, but take the bigger one (3^)

LCM = 3^ x 5 = 45 min

1 : 25 + 0 : 45 = 2 : 10

In this question you need to find the lowest common multiple. I like to do it the factor tree way:

15 9

/ / / /

3 5 3 3

See how there are both a 3 in the 15 factor tree and there is at least on three in the 9 factor tree. So when you find the LCM, you combine the two threes from the different factor trees like this:

3 x 5 x 3 = 45

So every 45 minutes the bus would be at the bus stop together

Now simply add the 45mins onto the 1:25

Answer: 2:10