A cylindrical drum of internal radius 21 cm and internal height 50 cm is filled to the brim from a tap that delivers 6.3 litres per minute. How many minutes does it take? Take π = 22/7.

Capacity = $(22/7)\times 21^2\times 50=1386\times 50=69300{\text{cm}}^3$, and $1000{\text{cm}}^3=1$ litre, so the drum holds 69.3 litres. Time = 69.3/6.3 = 11 minutes. (Dividing the $69300{\text{cm}}^3$ by 6.3 without turning it into litres gives an absurd 11000 minutes; using the diameter 42 cm as the radius makes the drum four times as big and gives 44 minutes; dividing by 63 instead of 6.3 gives 1.1 minutes.)