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

The correct answer is 9. This is a work and time problem, best solved using the "man-days" concept.

• Step 1: 21 binders bind 1400 books in 15 days.
• Total man-days = 21 × 15 = 315 man-days for 1400 books.
• Man-days per book = 315 ÷ 1400 = 0.225 man-days per book.
• Step 2: For 800 books, total man-days required = 800 × 0.225 = 180 man-days.
• Available days = 20.
• Number of binders required = 180 man-days ÷ 20 days = 9 binders.

Options 7, 12, and 14 are incorrect. Hence, 9 binders are required.