A cylindrical water tank of internal radius 70 cm stands full, with water 3 m deep. It is drained through a pipe of internal bore 7 cm, the water leaving at 4 metres per second. How many minutes does the tank take to empty? (Use π = 22/7.)

Work in centimetres and seconds. The pipe'$s$ bore is 7 cm across, so its radius is 3.5 cm and its cross-section is $(22/7)\times 3.5^2=38.5$ sq cm; at 4 m/$s=400$ cm/$s$ the outflow is $38.5\times 400=15400$ cubic cm every second. The tank holds (22/7) $\times 70^{2}x$ 300 = 4620000 cubic cm, so it empties in 4620000/15400 = 300 seconds = 5 minutes. The trap 1.25 reads the pipe'$s$ 7 cm as its RADIUS, making the cross-section four times too large; 50 converts 4 m/$s$ to 40 cm/$s$ instead of 400 cm/$s$; 300 is the correct figure in seconds, quoted as though it were minutes.