‘A’ completes a work in 12 days. ‘B’ completes the same work in 15 days. ‘A’ started working alone and after 3 days B joined him. How many days will they now take together to complete the remaining work?  Answers
A) 5  B) 8  C) 6  D) 4  E) none of these

Rationale: work done by ‘ a ’ in 3 days = 1 ⁄ 12 × 3 = 1 ⁄ 4 ∴ remaining work = 1 - 1 ⁄ 4 = 3 ⁄ 4 work done by a and b together = 12 × 15 / 27 = 20 / 3 ∴ remaining work done by a and b together in = 3 ⁄ 4 × 20 ⁄ 3 = 5 days answer a
Answer: A