A pipe of diameter 'd' can drain a water tank in 40 minutes. The time taken by a pipe of diameter '2d' for doing the same job is:

10 min

Note: Ratio of efficiency of the pipes is always equal to the square of the radius of the pipes.

                     I     :     II

Diameter :    D    :    2D

Radius    :    1     :     2

Efficiency:    1²   :     2²

Time       :    4     :     1

Here,

4R = 40 min,

1R = 10 min.