Mrs. SImpson waters her geraniums once every 3 days, and she waters her other house plants once a week. How often does mrs. Simpson water all the plants on the same day?

She often waters all the plants on the same day every 21 days

Step-by-step explanation:

- Mrs. Simpson waters her geraniums once every 3 days

- She waters her other house plants once a week

∵ Each week has 7 days

∴ She waters her other house plants once every 7 days

- We need to know How often does Mrs. Simpson water all the plants

on the same day

* Lets think about that:

∵ She waters her geraniums once every 3 days

- That mean 3, 6, 9, 12, ...

∴ She will water her geraniums in a multiple of 3

∵ She waters her other house plants once every 7 days

- That mean 7, 14, 21, 28, ...

∴ She will water other house plants in a multiple of 7

- That means she will water both in a common multiples of 3 and 7

∴ She will water all in a common multiple of 3 and 7

∵ The first common multiple of 3 and 7 is 21

∴ Mrs. Simpson waters all the plants on the same day every 21 days

* She often waters all the plants on the same day every 21 days