A pipe can fill a pool in 12 hours. Another pipe can fill the pool in 18 hours. How long will it take for the two pipes to fill the pool if they operate simultaneously?

1 answer

let T equal the fill time ... each pipe fills a fraction of the pool
... the fractions sum to one (the whole pool)

T/12 + T/18 = 1

find a common denominator ... 3T/36 + 2T/36 = 1

3T + 2T = 36