21 binders can bind 1400 books in 15 days. How many binders will be required to bind 800 books in 20 days? · ABC26GN3357

Let the required number of binders be $x$. Less books, Less binders (Direct Proportion). More days, Less binders (Indirect Proportion). Books $1400:800$, Days $20:15$ :: $21:x$. $\therefore 1400 \times 20 \times x = 800 \times 15 \times 21 \Leftrightarrow x = \frac{800 \times 15 \times 21}{1400 \times 20} = 9$.