A hall has 800 seats and a show is called houseful only if at least 90% of the seats are occupied. Tickets were sold for 750 seats, but 12% of the ticket-holders did not turn up. By what per cent of the hall's seats did the attendance fall short of the houseful mark?

The houseful mark is 90% of 800 = 720. Those present = $750\times 0.88=660$, so the shortfall is 60 seats. The phrase 'of the hall's seats' fixes 800 as the base: 60/800 = 0.075 = 7.5%. 8% is 60/750, measured against the tickets sold. 12% is the no-show rate, which is a share of the ticket-holders, not of the seats. 82.5% is 660/800, the attendance itself rather than the shortfall.