Correct Answer - Option 2 : 36 hours
Given :
A pipe can fill a cistern in 12 hours and another pipe can empty it completely in 18 hours.
Concept used :
Total time taken = Total work/Work per unit time
Calculations :
Total work = LCM of 12 and 18 = 36 units
Work done by Fill type per hour
⇒ 36/12 = 3 units
Work done by empty pipe
⇒ 36/18 = - 2 units (-ve sign as they are working in opposite direction)
Total work per hour by them
⇒ 3 - 2 = 1 unit
Time taken by both pipes to complete the work = 36/1
⇒ 36 hours
⇒ 36 The cistern will be filled in 36 hours.
