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 ?

\par One day's work of the three persons = \frac{1}{15}+\frac{1}{20}+\frac{1}{25} = \frac{47}{300}

\par So, all the three together will complete the work in \frac{300}{47} = 6.4  \par days