Three inlets that alone would fill a tank in 30, 20 and 60 minutes are opened together on an empty tank and left running until the tank is full. What fraction of the tank is filled by the slowest of the three?

Take the tank as 60 units: the inlets deliver 2, 3 and 1 units a minute, a total of 6. All three run for the same length of time, so the tank is shared in the ratio of the rates 2 : 3 : 1, and the slowest inlet supplies 1/6 of it. 5/6 is the share of the other two together, the complement of the answer. 1/3 splits the tank equally among three inlets of unequal delivery. 6/11 shares it in the ratio of the solo times 30 : 20 : 60, which hands the biggest share to the weakest inlet.