Shortcut Trick
Efficiency of A and B are in ratio 36 : 18 = 2 : 1.
Total work = 36 units (LCM of 18 and 36).
Combined work done in 8 days = (2 + 1) × 8 = 24 units.
Remaining work = 36 − 24 = 12 units.
Days taken by B to finish remaining work = 12 ÷ 1 = 12 days.
∴ The correct answer is 12 days.
Alternate Method
Given: A can do the work in 18 days and B in 36 days. They work together for 8 days.
| Person | Days | Total Work | Efficiency |
|---|
| A | 18 | 36 | 36 ÷ 18 = 2 |
| B | 36 | 36 ÷ 36 = 1 | |
⇒ Efficiency of (A + B) = 2 + 1 = 3 units/day
⇒ Work done by A and B in 8 days = 3 × 8 = 24 units
⇒ Remaining work after A leaves = Total Work − Work Done
⇒ Remaining work = 36 − 24 = 12 units
⇒ Time taken by B to complete remaining work = Remaining Work ÷ Efficiency of B
⇒ Time = 12 ÷ 1 = 12 days
∴ The correct answer is 12 days.
Additional Information
Fundamental Formula
Total Work = Efficiency × Time. This is the core principle for all Time and Work problems.
Efficiency Relation
Efficiency is inversely proportional to the time taken to complete a task. If the ratio of time is a : b, the ratio of efficiency is b : a.
Leaving/Joining Concept
When a person leaves, the remaining work is calculated and then divided by the efficiency of the person(s) still working to find the extra time required.