Multiple choice

A and B can do a job alone in 12 days and 15 days respectively. A starts the work and after 6 days B also joins to finish the work together. For how many days B actually worked on the job? A और B एक जॉब का क्रमशः 12 और 15 दिनों में कर सकते हैं। A काम करना प्रारम्भ करता है और 6 दिनों बाद B सम्मिलित होकर एक साथ काम को समाप्त करता है। B ने जॉब में वास्तविक रूप से कितने दिन भाग लिया?

  1. 3 1 3 days

  2. 2 5 7 days

  3. 5 days

  4. 6 days
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A Correct answer
Explanation

A's work rate is 1/12 per day, B's is 1/15 per day. Together they do 1/12 + 1/15 = 9/60 = 3/20 per day. A works alone for 6 days, completing 6/12 = 1/2 of the work. The remaining 1/2 is completed by A and B together at 3/20 per day, which takes (1/2) / (3/20) = 10/3 = 3 1/3 days. This is how long B worked. Option D (6 days) would be correct if B had worked from the start.

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