A housing society has 1,600 members, and its general body meeting is valid only if 45% of the members are present. Just 648 members turned up. The turnout was what per cent of the number needed?

The number needed is 45% of 1,600 = 720. The words 'of the number needed' fix 720 as the base: 648/720 = 0.90 = 90%. 40.5% is 648/1600, the turnout as a share of ALL members - a different question, and the commonest slip here. 10% is the shortfall share, the opposite of what was asked. 111.11% is 720/648, the comparison turned round.