Question

you have reading homework every 6 days and you have science homework every 15 days. If you have homework in both today when will you have homework in both subjects on the same day again?

Answer 1

Do you remember when you were learning about the "Least Common
Multiple" of two numbers, and you were wondering why you had to learn
it, because you were so sure that you'd never need to use it ?  Do you
remember that ?

Well, HERE's where you get to use it.  This question is just looking for
the LCM of 6 and 15 .  THAT's how many days it will be before you get
homework in both subjects on the same day again.

(It's not 90 days, and it's not 60 days.)