A pipe that fills a tank in 12 hours is opened on the empty tank, and at the end of every hour one more identical pipe is opened (2 pipes in the second hour, 3 in the third, and so on). When is the tank full?

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4.4 h. In 12ths, hours 1–4 add 1, 2, 3, 4 → 10. In hour 5 five pipes add 5 per hour; the last 2 take 0.4 h. Total 4.4 h.

A pipe that fills a tank in 12 hours is opened on the empty tank, and at the end of every hour one more identical pipe is opened (2 pipes in the second hour, 3 in the third, and so on). When is the tank full?

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