Two pipes can fill a tank in 20 minutes and 30 minutes respectively. Both pipes are opened simultaneously. When the tank should have been full, it is found that the waste pipe was open.Then the waste pipe was closed and in another 4 minutes, the tank was full. In what time would the waste pipe empty it.
35 minutes
36 minutes
37 minutes
38 minutes
Explanation:
The first two pipes together can fill the tank in $\dfrac{20×30}{20+30}$=12 minutes.
Therefore, quantity filled by first pipe and second pipe together in 4 minutes is equal to quantity emptied by waste pipe in 12 minutes.
i.e., quantity filled by first pipe and second pipe together in 12 minutes is equal to quantity emptied by waste pipe in 36 minutes.
i.e., waste pipe can empty the tank in 36 minutes.
The first two pipes together can fill the tank in $\dfrac{20×30}{20+30}$=12 minutes.
Therefore, quantity filled by first pipe and second pipe together in 4 minutes is equal to quantity emptied by waste pipe in 12 minutes.
i.e., quantity filled by first pipe and second pipe together in 12 minutes is equal to quantity emptied by waste pipe in 36 minutes.
i.e., waste pipe can empty the tank in 36 minutes.