In a housing society, 384 flats have covered parking and 320 do not. The flats with covered parking are what per cent more than the flats without it?

The excess is 384 - 320 = 64, and the words 'more than the flats without' fix 320 as the base: 64/320 = 0.20 = 20%. 16.67% uses 384 as the base (64/384) - the wrong base. 120% is 384/320, which reports the whole rather than the excess, and 83.33% is 320/384, the comparison turned the other way round.