Shortcut Trick
Total Work (LCM of 18, 24, 30) = 360 units.
Efficiencies: P = 20, Q = 15, R = 12.
3-day cycle: Day 1 (P) + Day 2 (P) + Day 3 (P+Q+R) = 20 + 20 + 47 = 87 units.
In 4 cycles (12 days): 87 × 4 = 348 units. Remaining = 360 − 348 = 12 units.
Time for remaining work = 12 ÷ 20 = 3/5 days.
Total time = 12 + 3/5 = 12 3/5 days.
∴ The correct answer is 12 3/5 days.
Alternate Method
Given: Time taken by P = 18 days, Q = 24 days, R = 30 days.
| Person | Days | Total Work | Efficiency |
|---|
| P | 18 | 360 | 20 |
| Q | 24 | 15 | |
| R | 30 | 12 | |
⇒ Efficiency of P = 360 ÷ 18 = 20 units/day
⇒ Efficiency of Q = 360 ÷ 24 = 15 units/day
⇒ Efficiency of R = 360 ÷ 30 = 12 units/day
⇒ Work done in Day 1 (P only) = 20 units
⇒ Work done in Day 2 (P only) = 20 units
⇒ Work done in Day 3 (P+Q+R) = 20 + 15 + 12 = 47 units
⇒ Total work in 1 cycle (3 days) = 20 + 20 + 47 = 87 units
⇒ Work done in 4 cycles (12 days) = 87 × 4 = 348 units
⇒ Remaining work = 360 − 348 = 12 units
⇒ Time taken by P to do remaining work = 12 ÷ 20 = 3/5 days
⇒ Total time = 12 + 3/5 = 12 3/5 days
∴ The correct answer is 12 3/5 days.
Additional Information
LCM Method for Time and Work
Total work is assumed as the LCM of individual time periods to ensure efficiencies are whole numbers. Efficiency = Total Work ÷ Time.
Alternate Day / Periodic Help Work
In periodic help problems, identify the repeating cycle (e.g., 3 days here) and calculate the total work done in one complete cycle.
Work and Wages Relation
Wages are always distributed in the ratio of the actual work done by each person, which is Efficiency × Time.