If A can do a work in 10 days and B can do the same work in 15 days, how long will it take for A to finish the work alone after B has worked for 3 days?
Answer: A. 5 days
B completes 3/15 = 1/5 of the work in 3 days. The remaining work is 4/5. A can complete the work in 10 days, so to finish 4/5 of the work, A will take (4/5)*10 = 8 days.