DRDO Multi tasking staff Model paper 9

Time taken by pipe 1 to fill = 4 hours
Time taken by pipe 2 to empty = 5 hours
Let capacity of cistern = LCM (4 and 5) = 20 units
Efficiency of pipe 1 = 20/4 = 5 units/hour
Efficiency of pipe 2 = 20/5 = –4 units/hour
Total Efficiency of pipe 1 and pipe 2 together = 5 – 4 = 1 unit/hour
⇒Total time taken to fill the cistern = 20/1= 20 hours