How do i find the difference between two different times e. g 13: 40 and 17: 15

You find their difference, of course!

E.g. 17:15 - 13:40 = 3:35/ 3 hrs 35 min.

Just remember that a full hour (1:00) is 60 minutes (0:60). You would never write anything as 0:60, though! Anything over 0:59 becomes 1:00 + the remaining number of minutes.

Hope this wasn't too confusing! Practice makes it easier to handle and understand, so try it out! Hope this helps :)

These are the multiples of 15 but what you are probably asking or what are the factors of 15