To do this, we need to find the LCM of 6 and 9. This can be found by listing out some multiples of each and then finding the first one they have in common:
Multiples of 6: 0, 6, 12, 18, 24, 30, 36
Multiples of 9: 0, 9, 18, 27, 36, 45
The first multiple they have in common is 18, so that is the amount of days until he will hike and swim on the same day. Note that both lists have 36, but this is not the answer since it is not the smallest multiple that they have in common.
