Shortcut Trick
Time taken by pipes A + B + C together = 40 hours.
Since A + B + C takes 4/5 more time than A + B, Time(A + B + C) = (1 + 4/5) × Time(A + B) = 9/5 × Time(A + B).
Time taken by A + B = 40 × (5/9) = 200/9 hours.
Let Total Work = LCM(40, 200/9) = 200 units.
Efficiency of C = Efficiency(A + B) − Efficiency(A + B + C) = 9 − 5 = 4 units/hour.
Time taken by C alone = 200 ÷ 4 = 50 hours.
∴ The correct answer is 50 hours.
Alternate Method
Given:
Time taken by pipes A, B, and C together = 40 hours.
Time taken by A, B, and C = Time taken by A + B + (4/5) × Time taken by A + B.
Formula Used:
Total Work = Time × Efficiency
Calculations:
⇒ Let the time taken by pipes A + B alone be T hours.
⇒ Time taken by A + B + C = T + 4T/5 = 9T/5 hours.
⇒ 9T/5 = 40 ⇒ T = 200/9 hours.
| Pipe | Hours | Total Work | Efficiency |
|---|
| A + B | 200/9 | 200 | 9 |
| A + B + C | 40 | 5 | |
⇒ The combined efficiency of A and B is 9 units/hour.
⇒ The combined efficiency of A, B, and C is 5 units/hour.
⇒ Since C is an outlet pipe, Efficiency of C = Efficiency(A + B) − Efficiency(A + B + C)
⇒ Efficiency of pipe C = 9 − 5 = 4 units/hour.
⇒ Time required by C to empty the tank alone = Total Work ÷ Efficiency of C
⇒ Time taken by C = 200 ÷ 4 = 50 hours.
(Note: The data stating A takes 4/5 of the time required by B is extra information. Time ratio 4:5 means efficiency ratio is 5:4, which perfectly sums to the 9 units of efficiency we calculated, making the data completely consistent.)
∴ The correct answer is 50 hours.
Additional Information
Pipes and Cisterns Principle
This concept operates identically to Time and Work. If a pipe fills a tank in x hours, the fraction of the tank filled in 1 hour is 1/x.
Outlet Pipe Concept
An outlet pipe removes water, so its efficiency is considered negative. When operating together, Total Efficiency = Inlet Efficiency − Outlet Efficiency.
Time and Efficiency Relationship
Time taken to complete a task is inversely proportional to efficiency. If the ratio of time taken by two pipes is a : b, their efficiency ratio is b : a.