It will take 16 days for Mary to complete a certain task alone. She worked for 8 days before she was joined by her sister. both of them completed the remaining task in 2 and half days. if her sister had joined her when she started the task, how many days would it have taken?	Pick one: A) 6 B) 8 C) 2 D) 5 E) 4

Rationale: explanation : mary and her sister complete half work in 2.5 days = > they can complete whole work in 5 days answer : option d
Answer: D