Three painters, all working at the same steady speed, paint 3 walls in 3 hours. How long would 9 such painters take to paint 9 walls?

Share the work out per painter: 3 painters finishing 3 walls in 3 hours means each painter finishes one wall in 3 hours. Give 9 painters a wall each and they all finish together after the same 3 hours. The trap is 9 hours, which comes from noticing that both numbers have tripled and tripling the time as well — but the extra painters and the extra walls cancel each other out exactly, so the time does not move at all. 27 hours triples twice over, and 1 hour divides where it should not.