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?
Answer: C
Let W be woman's work, C be child's work. From the problem, \(10W \times 7 = 10C \times 14 \Rightarrow 70W = 140C \Rightarrow W=2C\). Total work = \(10W \times 7 = 10(2C) \times 7 = 140C\). We need time for 5 women and 10 children. Their combined rate = \(5W+10C = 5(2C)+10C = 10C+10C=20C\). Time = Total work / Rate = 140C / 20C = 7 days.