UPSC CAPF Assistant Commandant Model Paper 7

GIVEN:
Pipe A can fill the cistern in 3 hours and pipe B can fill the cistern in 4 hours.
CONCEPT:
Total work = Time × Efficiency
CALCULATION:
Suppose total capacity of cistern = 12 units (LCM of 3 and 4)
So,
Efficiency of pipe A = 12/3 = 4 units
Efficiency of pipe B = 12/4 = 3 units
So,
A starts first followed by B then by A.
So, 4 + 3 + 4 = 11 units of work is completed in 3 hours.
Remaining 1 unit of work will be completed by B in 1/3 hours.
∴ Total time = 3 + 1/3 = 10/3 hours