Time Work

A tank has an inlet pipe and an outlet pipe. If the outlet pipe is closed then the inlet pipe fills the empty tank in 8 hours. If the outlet pipe is open then the inlet pipe fills the empty tank in 10 hours. If only the outlet pipe is open then in how many hours the full tank becomes half-full?

  1. 20
  2. 30
  3. 40
  4. 45

Answer

The inlet pipe can normally fill the tank in 8 hours, but it takes 10 hours when the outlet pipe is also open. In the two additional hours, the inlet pipe fills 2/8 or 25% more, and the outlet pipe took 10 hours to drain the additional 25%.

So, the outlet pipe can drain 25% in 10 hours, and to drain 50%, it’ll take 20 hours.

Alternately, you can assume total capacity of the tank as LCM [8, 10] or 40 litres. You know the filling rates and the emptying rates of both the pipes and you can answer the question based on the same. 

The correct option is A.