Water flows at 5 km per hour through a pipe of internal diameter 14 cm into a rectangular tank 50 m long and 44 m wide. How long will it take for the water level in the tank to rise by 7 cm? (Use π = 22/7.)

The pipe's radius is 7 cm = 0.07 m, so its cross-section = $(22/7)\times 0.07^2=0.0154$ sq m. At 5 km/$h=5000$ m/$h$ the pipe delivers $0.0154\times 5000=77$ cubic m each hour. The tank needs $50\times 44\times$ 0.07 = 154 cubic m, so the time = 154/77 = 2 hours. The trap 30 minutes uses 14 cm as the radius, which quadruples the flow; 20 hours writes the 7 cm rise as 0.7 m; 200 hours writes it as 7 m.