One pipe fills a store pool in 12 hours. A second pipe fills the same pool in 6 hours. When a third pipe is added and all three are uset otill the pool, it only takes 3 hours. Find how long it takes the third pipe to do the job

x  = 12  hours

Step-by-step explanation:

If one pipe fills store pool in 12 hours   in 1 hour will fill 1/12 of the pool

If one pipe fills a store pool in 6 hours in 1 hour will fill 1/6 of the pool

Let call " x " number of hours needed by the third pipe to fills the pool

then in 1 hour  will fill 1/x

The three pipes working together in 1 hour will fill

1/12  +  1/6  + 1/x

Now we know that the three pipes take 3 hours to fill the pool then in 1 hour three pipes will fill 1/3 of the pool, therefore

1/12  +  1/6  + 1/x  = 1/3

( x + 2*x + 12) /12*x  = 1/3   ⇒ ( 3*x +12 )/ 12*x = 1/3

9*x  + 36  = 12*x

- 3*x  = - 36

x  = 36/3

x  = 12  hours

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step-by-step explanation: