'A' can do a piece of work in 30 days and ' B ' alone can do it in 20 days. They work together for 4 days and then ' B ' leaves the job. 'A' will finish the remaining work in

Correct option is A

Given
A's time = 30 days
B's time = 20 days

Formula Used
Efficiency = $$\frac{\text{Total Work}}{\text{Time}}$$​​

Solution
Total Work = LCM(30, 20) = 60 units
A's efficiency = $$\frac{60}{30} = 2$$​units/day

B's efficiency = $$\frac{60}{20}$$​= 3 units/day
Work done in 4 days together = $$4 \times (2+3)$$​ = 20 units
Remaining work = 60 - 20 = 40 units
Time taken by A to finish = $$\frac{40}{2}$$​ = 20 days

Final Answer
So the correct answer is (a)