1. The problem statement, all variables and given/known data The problem is as follows " 2 pipes fill cistern with water in 4 and 6 hours respectively.If the first pipe in 4 and 6 hours respectively . if the first pipe be opened first and the pipes be opened alternatively one at a time for 1 hour each, in how many hours will the cistern be filled up" My attempt: I first took that in 1 hour first pipe fills 1/4th of tank and second pipe 1/6 thof tank .: in 2 hours they fill 1/4+1/6= 5/12th of tank in 1hour they fill 5/24 th of tank .: they fill the tank in 24/5 hours But the problem is my book's answer is 19 hours.I don't know how that is possible because if they alone fill it in 4 and 6 hours then if they are working together the time should be less. Please help me!