Pipes A and B fill a tank in 18 and 27 hours. Both run together, except that every third hour B is shut for that hour (hours 3, 6, 9, …). How long does the empty tank take to fill?

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12.4 h. In 54ths: both give 5 per hour, A alone 3. A 3-hour block gives 5+5+3=13. After 4 blocks (12 h): 52; in hour 13 both run and need 2 of their 5 → 0.4 h. Total 12.4 h. (10.8 h has B open throughout.)

Pipes A and B fill a tank in 18 and 27 hours. Both run together, except that every third hour B is shut for that hour (hours 3, 6, 9, …). How long does the empty tank take to fill?

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