A pipe of diameter d can drain a certain water tank in 40 minutes. The time taken by a pipe of diameter 2d for doing the same job? एक पाइप जिसका व्यास d है टंकी को 40 मिनट में खली करता है तो दूसरा पाइप इसी टंकी को कितने मिनट में खाली करेगा जिसका व्यास 2d है?

The rate of drainage is proportional to the cross-sectional area of the pipe, which depends on the square of the radius (or diameter). When diameter doubles from d to 2d, the area increases by factor of 4, so the drainage rate becomes 4 times faster. Therefore, time becomes 40/4 = 10 minutes. The claimed answer is correct.