Explanation

21 binders bind 1400 books in 15 days, so 1 binder binds:

1400/21 = 66.67 books in 15 days

= 66.67/15 = 4.44 books per day

To bind 800 books in 20 days, we need:

800/20 = 40 books per day

Number of binders required = 40/4.44 = 9

So, 9 binders are required to bind 800 books in 20 days.