A leaves mid-way; B finishes the remainder A can do a job in 10 days and B in 20 days. They start together, but after 5 days A leaves. In how many days will B complete the remaining work?

Introduction / Context:
When one worker leaves part-way, compute the completed fraction so far, then divide the remainder by the staying worker’s rate.

Given Data / Assumptions:

• A: 10 days ⇒ 1/10 per day.
• B: 20 days ⇒ 1/20 per day.
• They work together for the first 5 days.

Concept / Approach:
Together rate = 1/10 + 1/20 = 3/20 per day. Remaining = 1 − (3/20)*5.

Step-by-Step Solution:

Verification / Alternative check:
Total job = 3/4 + (1/4) = 1. Timings are consistent.

Why Other Options Are Wrong:
6, 8, 10 contradict B’s fixed rate of 1/20 for a quarter job.

Common Pitfalls:
Adding 5 directly to an option instead of calculating the leftover fraction properly.

Final Answer:
5 days