21 binders can bind 1400 books in 15 days. How many binders will be required to bind 800 books in 20 days?
(a)7
(b)9
(c)12
(d)14
(e)None of these
Answer
Answer (as printed): B
Explanation
Let the required number of binders be $x$. Less books, Less binders (Direct Proportion). More days, Less binders (Indirect Proportion). Books $1400:800$, Days $20:15$ :: $21:x$. $\therefore 1400 \times 20 \times x = 800 \times 15 \times 21 \Leftrightarrow x = \frac{800 \times 15 \times 21}{1400 \times 20} = 9$.
Explanation as extracted from the printed page; notation may be imperfect.