A hall is 15 m long, 10 m wide and 4 m high. Its four walls and its ceiling are to be distempered, but not the floor. One door measuring 2.5 m by 1.6 m and two windows each 2 m by 1.5 m are left out. If one litre of distemper covers 10 square metres, how many litres are needed?

The four walls give $2(15+10)\times 4=200$ sq m and the ceiling $15\times 10=150$ sq m, so 350 sq m before deductions. The door is 4 sq m and the two windows are 3 sq m each, i.e. 10 sq m in all, leaving 340 sq m. At 10 sq m per litre that is 34 litres. The trap 35 paints straight over the door and windows; 19 deducts correctly but forgets the ceiling, using only 190 sq m of wall; 36 ADDS the 10 sq m of openings instead of taking them off.