Answer :
The "least common multiple" of 4 and 7 is 28. Beware of anybody who tells you to just multiply them. The least common multiple is not always their product. For example, the least common multiple of 4 and 6 is 12, NOT 24.
The correct answer is:
The 28th day, and every 28 days after.
Explanation:
We want to find the least common multiple (LCM) of 4 and 7 to solve this. To find the LCM, we find the prime factorization of each number:
4 = 2*2
7 = 1*7
We look for common factors; there are none. We multiply the uncommon factors (in this case, all of them): 2*2*7 = 28
This means the first day we water both is day 28. Every 28 days we will water both plants on the same day.