A and B undertake a project worth Rs. 54000. A alone can do the work in 10 days. They work together for 3 days. After 3 days, B works alone for 3 days and A completes the remaining work in 3 more days. What is the share of B in the earnings?

Let total work = 1 unit. A can complete in 10 days, so A's rate = 1/10 per day. A works for 3+3=6 days total, completing 6/10 = 3/5 of work. B works for 3+3=6 days, completing 2/5 of work. So B's rate = (2/5)/6 = 1/15 per day, meaning B alone would take 15 days. Earnings are split in ratio of work done: A:B = 3/5:2/5 = 3:2. B's share = (2/5) × 54000 = Rs. 21600, which matches option A.