Five pipes fill a tank in 8 hours, now four pipes, three tanks

Question

Five identical pipes fill one tank in 8 hours. Solve in your head. How long do four pipes take to fill three tanks? Aim: under 15 seconds.

Accepted Answer

Convert everything to pipe-hours, the invariant unit of work: - One tank costs 5 	imes 8 = 40 pipe-hours. - Three tanks cost 120 pipe-hours. - Four pipes supply 4 pipe-hours per hour, so \frac{120}{4} = 30 	ext{ hours}. Takeaway. Name the conserved quantity (pipe-hours) and the problem becomes two divisions: total work, then work per hour. Routing through total work sidesteps the classic trap of scaling time the wrong way when the number of pipes drops.

Five pipes fill a tank in 8 hours, now four pipes, three tanks

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