The elementary school lunch menu repeats every 20 days; the middle school lunch menu repeats every 15 days. Both schools are serving hamburgers today. In how many days will BOTH schools serve hamburgers again

Answer 1

This is like finding the Lowest Common Multiple of 20 and 15, which is 60
So it would happen in 60 days.

or

days elementary lunch repeats:
20, 40, 60, 80, ...
days middle lunch repeats:
15, 30, 45, 60, 75, ...

which number shows up first in both sequences ?