Shortcut Trick
Total Capacity (LCM of 15, 30, 40) = 120 units.
Efficiency: A = 8, B = 4, C = 3.
Work done by A (6 am to 10 am = 4 hrs) = 8 × 4 = 32 units.
Work done by B (8 am to 10 am = 2 hrs) = 4 × 2 = 8 units.
Remaining work at 10 am = 120 − (32 + 8) = 80 units.
Time from 10 am = 80 ÷ (8 + 4 + 3) = 80 ÷ 15 = 5 hours 20 minutes → 3:20 p.m.
∴ The correct answer is 3:20 p.m.
Alternate Method
Given: Filling times: A = 15 hrs, B = 30 hrs, C = 40 hrs. Opening times: A = 6 a.m., B = 8 a.m., C = 10 a.m.
Formula Used: Work = Time × Efficiency | Efficiency = Total Work ÷ Individual Time
| Person | Days | Total Work | Efficiency |
|---|
| Pipe A | 15 | 120 | 8 |
| Pipe B | 30 | 4 | |
| Pipe C | 40 | 3 | |
⇒ Between 6 a.m. and 8 a.m. (2 hrs), only A is open.
⇒ Work done by A = 8 × 2 = 16 units.
⇒ Between 8 a.m. and 10 a.m. (2 hrs), A and B both are open.
⇒ Work done by A + B = (8 + 4) × 2 = 12 × 2 = 24 units.
⇒ Total work completed by 10 a.m. = 16 + 24 = 40 units.
⇒ Remaining work to be done after 10 a.m. = 120 − 40 = 80 units.
⇒ After 10 a.m., all three pipes A, B and C are open. Combined efficiency = 8 + 4 + 3 = 15 units/hr.
⇒ Additional time taken = 80 ÷ 15 = 5.33 hours = 5 hours (0.33 × 60) mins = 5 hours 20 minutes.
⇒ Final time = 10:00 a.m. + 5 hours 20 minutes = 3:20 p.m.
∴ The correct answer is 3:20 p.m.
Additional Information
Pipes and Cisterns - Negative Work
If an outlet pipe (leak) is present, its efficiency is taken as negative. Total efficiency = ∑(Filling efficiencies) − ∑(Emptying efficiencies).
Alternate Filling
If pipes are opened for 1 hour each in turn, calculate the work done in one full cycle (sum of efficiencies) and then find the number of cycles to reach total work.
Total Capacity Rule
Taking the LCM of individual times as the total capacity of the tank simplifies calculations by converting fractional work into whole number units of efficiency.
Fractional Time Conversion
To convert decimal or fractional hours to minutes, multiply the fractional part by 60. Example: 0.33 hours = 1/3 × 60 = 20 minutes.