You would look for the LCM of both the numbers and that would be the # of days until she will do both on the same day. in this case it would be 30.
Or if you want an easier way, here it is. if both or one of the numbers are prime sometimes multiplying them together will get you the answer as well.
It works in this case: 15 x 2 = 30
30 is your answer
hope this helps!