Shortcut Trick
Total Work = Efficiency of A × Days taken by A = 2 × 40 = 80 units.
Work done by A, B, and C together in 5 days = (2 + 3 + 5) × 5 = 50 units.
Remaining work = 80 − 50 = 30 units.
Time taken by A and B to complete remaining work = 30 ÷ (2 + 3) = 6 days.
∴ The correct answer is 6 days.
Alternate Method
Given:
Efficiency ratio of A : B : C = 2 : 3 : 5
A alone can complete the work in 40 days.
Formula Used:
Total Work = Efficiency × Time
Time = Total Work ÷ Efficiency
Calculations:
⇒ Let the efficiencies of A, B, and C be 2 units/day, 3 units/day, and 5 units/day respectively.
⇒ Total Work = Efficiency of A × Time taken by A = 2 × 40 = 80 units.
⇒ Time taken by B = 80 ÷ 3 days.
⇒ Time taken by C = 80 ÷ 5 = 16 days.
| Person | Days | Total Work | Efficiency |
|---|
| A | 40 | 80 | 80 ÷ 40 = 2 |
| B | 80/3 | 80 ÷ (80/3) = 3 | |
| C | 16 | 80 ÷ 16 = 5 | |
⇒ Combined efficiency of A, B, and C = 2 + 3 + 5 = 10 units/day.
⇒ Work completed by A, B, and C in 5 days = 10 × 5 = 50 units.
⇒ Remaining work = Total Work − Completed Work = 80 − 50 = 30 units.
⇒ Combined efficiency of A and B = 2 + 3 = 5 units/day.
⇒ Time required by A and B to finish the remaining work = Remaining Work ÷ Efficiency of (A + B)
⇒ Time required = 30 ÷ 5 = 6 days.
∴ The correct answer is 6 days.
Additional Information
Efficiency and Time Relationship
Efficiency is inversely proportional to the time taken to complete a given work. If the efficiency ratio of two workers is x : y, then their time taken ratio will be y : x.
Work and Wages
Wages are always distributed in proportion to the total work done by each individual. If they work together for the same number of days, the wages are divided strictly in the ratio of their efficiencies.
Alternate Day Work
If persons work on alternate days, treat one complete cycle (e.g., A on day 1, B on day 2) as a block. Find the work done in one block and divide the total work to find the number of full cycles before checking the remaining units.
Negative Work (Pipes & Cisterns)
If a person destroys work or a waste pipe empties a tank, their efficiency is assigned a negative value. Net Efficiency = Sum of positive work rates − Sum of negative work rates.