Question

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

Answer 1

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

Answer 2

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