Question
In what time would a cistern be filled by three pipes
whose diameter are 2cm, 2*2/3cm, 4cm running together, when the largest pipe alone fill the cistern in 122 minutes the amount of water flowing in  by each pipe being proportional to the square of diameter?Solution
Total Efficiency = 9 + 16 + 36 = 61 Pipe 3rd alone can fill the cistern in 122 min (given) ∴ Capacity of the tank = (36 × 122) = 4392 Time taken by all pipes together = 4392/(61 )= 72 minutes