A man can do a job in 15 days. His father takes 10 days and his son finishes it in 25 days. How long will they take to complete the job if they all work together?  Choose the correct choice from the following choices:
A) 6.3
B) 6.9
C) 7.1
D) 6.1
E) 6.2
1 day work of the three persons = (1/15 + 1/10 + 1/25) = 21/150   So, all three together will complete the work in 150/21 = 7.1 days.   Answer:7.1
<result>C</result>
A man can do a job in 15 days. His father takes 10 days and his son finishes it in 25 days. How long will they take to complete the job if they all work together?