So a particularly dumb client has decided that before I can work for them I have to pass a high school maths test. Never having had to do US school tests I looked at some practice questions, including: "Two identical taps fill 2/5 of a tank in 20 minutes. When one of the taps goes dry in how many minutes will the remaining one tap fill the rest of the tank ?" Brilliantly ambiguous question If they mean that two taps together have filled 2/5 (40%) of the tank in 20mins, then each fills 20% in 20mins, ie 1%/min and so one will take 60mins to fill the remaining 60%. If they mean that each of the taps alone fill 2/5 (40%) in 20mins, then each fills 2%/min and then one will fill the remaining 20% in 10min. Or if you assume they are connected to the same water supply so that when one stops the other doubles in flow then you have either 30mins or 5mins. The 'correct' answer apparently is 15mins?