40 men can complete a piece of work in 15 days. 20 more men join them after 5 days they start doing work. How many days will be required by them to finish the remaining work? a. `7 2/3\ d a y s` b. `6 1/5\ d a y s` c. `8 1/4\ d a y s` d. `6 2/3\ d a y s`