SOLUTION: A pool can be filled by one pipe in 7 hours and by a second pipe in 9 hours. How long will it take using both pipes to fill the pool

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Question 715261: A pool can be filled by one pipe in 7 hours and by a second pipe in 9 hours. How long will it take using both pipes to fill the pool
Answer by josgarithmetic(39618) About Me  (Show Source):
You can put this solution on YOUR website!
"Fill the pool" is 1 job.

Pipe_____________rate______________time_______________jobs
Pipe One_________(____)____________7__________________1
Pipe Two_________(____)____________9__________________1
Both Pipes_______(_____)__________(___)_______________1

Fill in the missing information. r*t=j, meaning r=j/t.

Pipe_____________rate______________time_______________jobs
Pipe One_________(1/7)____________7__________________1
Pipe Two_________(1/9)____________9__________________1
Both Pipes_______(1/7+1/9)__________(___)_______________1

RATE means how many jobs per hour. The combined rate of both pipes together only needs to be computed from the sum.

Combined Rate, 1%2F7%2B1%2F9=%289%2B7%29%2F63
=highlight%2816%2F63%29 JOBS PER HOUR.

What the question asks is how many hours to do the job which is to "fill the pool." We just need the reciprocal of this combined rate. That is highlight%2863%2F16%29 hours.