A man can do a job in 5 days. His father takes 20 days and his son finishes it in 25 days. How long will they take to complete the job if they all work together?
Select the correct option.
A) 6.3  B) 6.9  C) 3.5  D) 6.1  E) 6.2

Rationale: "1 day work of the three persons = ( 1 / 5 + 1 / 20 + 1 / 25 ) = 29 / 100 so , all three together will complete the work in 100 / 29 = 3.5 days . answer : c"
Answer: C