A water board finds that 0.35% of the water it sends out leaks away before reaching homes. In every 1,000 litres sent out, how much water leaks away?

0.35% of $1,000=0.0035\times 1000=3.5$ litres. Going the other way, 3.5 per thousand is 0.35 per hundred, which is exactly what 0.35% means. 0.35 litres treats the per cent figure as though it were already quoted per thousand. 35 litres is 3.5% of the supply, one decimal place adrift, and 350 litres is 35%.