A leaking tap loses 0.25 litre of water every hour. How much water (in litres) leaks in 4.5 hours?

Ignore the points: $25\times 45=1125$. Total decimal places = 2 + 1 = 3, so the answer is 1.125 litres. Placing the point one spot too far right gives 11.25 — and a quick sense check kills it, since a quarter-litre an hour cannot fill 11 litres in under 5 hours.