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

Answer: Option B

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