Three printers, 20 minutes, one batch, now five printers, four batches

Question

Three identical printers finish one print batch in 20 minutes. Solve in your head. How long do five printers take to finish four batches? Aim: under 15 seconds.

Accepted Answer

Use printer-minutes as the invariant unit of work: - One batch costs 3 	imes 20 = 60 printer-minutes. - Four batches cost 240 printer-minutes. - Five printers supply 5 printer-minutes per minute, so \frac{240}{5} = 48 	ext{ minutes}. Takeaway. More machines and more output at once is no harder: total the work in printer-minutes, then divide by the machines' combined rate. The two clean divisions absorb both changes and keep fractions out of your head.

Three printers, 20 minutes, one batch, now five printers, four batches

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