A colony's water tank holds 40,000 litres when full, and the rule is that the level must never drop below 30% of the tank's capacity. The tank was full on Monday and 22,000 litres have since been drawn. What per cent of the tank's capacity may still be drawn before the limit is reached?

The floor is 30% of 40,000 = 12,000 litres. The tank now holds 40,000 - 22,000 = 18,000 litres, so only 18,000 - 12,000 = 6,000 litres may still be drawn: 6000/40000 = 0.15 = 15%. Shortcut: 100 - 55 - 30 = 15. 45% is all that is left in the tank, ignoring the floor that must be kept. 30% quotes the floor itself, and 55% is what has already been drawn.