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?
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