A hostel cook uses 35% of a 40 kg sack of flour on Monday, and on Tuesday he uses 4 kg less than he used on Monday. What per cent of the sack is left after Tuesday?

Monday: 35% of 40 = 14 kg. Tuesday: 14 - 4 = 10 kg. Used in all = 24 kg, so 16 kg is left, and 16/40 = 0.40 = 40%. 60% is the share used, the opposite of what was asked. 34% reads '4 kg less' as '4 percentage points less', making Tuesday 31% and the total used 66%. 30% assumes Tuesday matched Monday at 35%, giving 70% used. Whenever a question mixes kilograms with per cents, put everything into one of the two before adding.