10 women can complete a work in 7 days and 10 children take 14 days to complete the work. How many days will 5 women and 10 children take to complete the work?

1 woman's 1 day's work = 1/70
1 Child's 1 day's work = 1/140
5 Women and 10 children 1 day work =      
                                                       $$\begin{gathered} \left(\frac{5}{70}+\frac{10}{140}\right) \\ =\frac{1}{7} \end{gathered}$$
So 5 women and 10 children will finish the work in 7 days.