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