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Steve & Jim are correct, but perhaps this can help you think about the relationship between the pumps & water:
If the first pump takes 12 minutes alone, and both pumps together do it in 3 minutes, then we know that the first pump empties 1/4 of the pool in the time it takes the second pump to empty 3/4 of the pool. Emptying 3/4 of a pool in 3 minutes is the same as 1/4 of the pool each minute. Therefore, the second pump does the job in 4 minutes.


Thanks to you all for helping me out, it looks real easy when you know what your doing.
Again many thanks
Pump 1: r1 = 1 pool/12 minutes
Pump 2: r2 = ?
1 pool = r1*3 + r2*3
r1 + r2 = 1/3
r2 = 1/3 - r1 = 1/3 - 1/12
r2 = 4/12 - 1/12 = 3/12
r2 = 1 pool/4 minutes
By itself, the second pool could drain the pool in 4 minutes.


            Let V=volume of the pool. Using the general rate and time equation V=R*T and applying it the the first instance gives
                        V=R1*12 so R1=V/12.  now let R2=pumping rate of the second pump so V=(R1+R2)*3 so R1+R2=V/3 we know what R1 is so R2=V/3 -V/12=3V/12 or V/4. If we now use this pump to empty the pool it will take V/Rminutes  or T=V/(3V/12) or 4 minutes to empty.
Hope this helps