if it takes one pipe 15 days to fill a large pool and takes another pipe 21 days to fill the same pool, how long will it take both pipes working together to fill the pool?

Answer 1

If pipe A fills the pool in 15 days, then in 1 day it filll 1/15th of the pool.

If pipe B fills the pool in 21 days, then in 1 day it fills 1/21st of the pool.

So, together they do 1/15 + 1/21, or 1/x of the pool in 1 day.

1/15 + 1/21 = 1/x

21/315 + 15/315 = 36/315 = 1/x

Cross-mult: 36x = 315 x = 315/36 = 8 27/36, reduces to 8 3/4 days to fill the pool if both pipes work together.