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

Explanation:-

Let the required number of binders be x
Less books, Less binders (Direct proportion)
More days, Less binders (Indirect proportion)
$$\left. \begin{gathered} {\text{Books 1400}}:800 \hfill \\ \,\,\,\,\,\,\,\,\,{\text{Days 20}}:18 \hfill \\ \end{gathered} \right\}::21:x$$
$$\eqalign{ & \therefore {\text{ }}1400 \times 20 \times x = 800 \times 15 \times 21 \cr & \Leftrightarrow x = \frac{{\left( {800 \times 15 \times 21} \right)}}{{\left( {1400 \times 20} \right)}} \cr & \Leftrightarrow x = 9 \cr}$$