Filling a pool with the drain open

MediumBrainteasers~4m

One hose fills a kiddie pool in 2020 minutes; another fills it in 1010 minutes. An open drain empties a full pool in 1515 minutes. With both hoses running and the drain open, how many minutes does it take to fill the empty pool?

Show hints (2)+
  1. Work in rates (pools/min): 120+110115\tfrac1{20}+\tfrac1{10}-\tfrac1{15}.
  2. Net =112=\tfrac1{12}, so 1212 minutes.

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Asked at: Akuna Capital, Flow Traders

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