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
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 ?