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1. 21 binders can bind 1400 books in 15 days. How many binders will required to bind 800 books in 20 days ?

A. 7

B. 9

C. 12

D. 14

Solution:

Let the required number of binders be x
Less books, Less binders (Direct proportion)
More days, Less binders (Indirect proportion)

\begin{matrix} Books 1400 : 800 \\ Days 20 : 15 \end{matrix}} :: 21 : x

\begin{aligned} ∴ 1400 \times 20 \times x = 800 \times 15 \times 21 \\ \Leftrightarrow x = \frac{(800 \times 15 \times 21)}{(1400 \times 20)} \\ \Leftrightarrow x = 9 \end{aligned}

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