Pipe and cistern important shortcut formulas

1.If two pipes A and B
A can fill a tank in x hrs and B can fill the same tank in y hrs
If both pipes are opened simultaneously ,then time taken to fill the tank is

Work done by both pipes together in 1hr=1/x +1/y

2.If two pipes A and B
A can fill a tank in x hrs and B can empty the same tank in y hrs
If both pipes are opened simultaneously ,then time taken to fill the tank is =

Work done by both pipes together in 1hr=1/x -1/y

3.If three pipes A ,B and C
A can fill a tank in x hrs,B can fill the same tank in y hr,and C takes z hrs for filling the same tank.
If three pipes are opened simultaneously ,then time taken to fill the tanks is

Work done by three pipes together in 1hr=1/x +1/y +1/z

4.If A can fill a tank in x hrs,B can fill the same tank in y hr and C takes z hrs for emptying the same tank.
If three pipes are opened simultaneously time taken to fill the tank is=

Work done by three pipes together in 1hr=1/x +1/y -1/z
hrs for filling.

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