Sophie has to water her aloe plant every 18 days and her spiral cactus every 30 days. if she waters both plants today, how long will it be until she waters plants on the same day again

both watered on the same day, If we multiply 18 by any number such as 2,3,4,5 we can find the next soonest date that they will both be watered on. 18*2,3,4,5 30,2,3,4,5

18*2,3,4 are not multiples of 30, 18*5 is 90

30*2 is not a multiple of 18, 30*3 is 90

Now we know the next closest date they will be watered again is 90 days from now.