Question 783272
One pipe can fill a pool in 9 hours and another pipe can fill the same pool in 12 hours. To fill up the pool, only the first pipe was turned on. After three hours it broke down, and the second pipe was automatically turned on.
a) How many hours did it take to fill up the pool?
b) Hom many hours would it take to fill up the pool if the two pipes started to work together?

pipe I does 1/9 of the job in 1hour

In 3 hours it does 3/9 =>1/3 of the job

Balance job = 1-1/3 = 2/3 of the job.


Pipe II does 1/12 of the job in 1 hour

to do 1/3 of the job it will take (1/3)/(1/12) = 4 hours

Total time = 3 + 4 = 7 hours

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Together they do 1/9 + 1/12 of the job in 1 hour

so they do 7/36 of the job in 1 hour

to complete the job they will take 36/7 hours

5.14 hours