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1. A wall of 100 meters can be built by 7 men or 10 women in 10 days. How many days will 14 men and 20 women take to build a wall of 600 metres ?

A. 15

B. 20

C. 25

D. 30

Solution:

Let the required number of days be x
7 men = 10 women
(14 men and 20 women) = (20 + 20) women = 40 women
More length, More days (Direct proportion)
More women, Less days (Indirect proportion)

\begin{matrix} Length 100 : 600 \\ Women 40 : 10 \end{matrix}} :: 10 : x

\begin{aligned} ∴ 100 \times 40 \times x = 600 \times 10 \times 10 \\ \Leftrightarrow x = \frac{(600 \times 10 \times 10)}{(100 \times 40)} \\ \Leftrightarrow x = 15 \end{aligned}

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