A man can do a job in 15 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?

A Less than 6 days

B Exactly 6 days

C More than 10 days

D Approximately 6.4 days

Correct Answer: Option D

In a day three of them do together do = (1/15+1/20/1/25)= 57/300 part of the work

57/300 part of the work in 1 day

so,total part of the work = 300/57 = 6.4 day(approx)