Some of helen's plants need water every day, some need water every other day, and others need water every third day. If she waters them all today, how many days will it be before she waters them all again?

Answer: Waters on 7th day

Step-by-step explanation:

If she waters everyday then days = 1,2,3,4,5,6,7,8,9

If she waters every other day = 1,3,5,7,9

If she waters every third day = 1,4,7,10,13

So the common day to all is either 1st day or 7th day.