If 18 bindres bind 900 books in 10 days , How many binders will be required to bind 660 books in 12 days?

Let the required no of binders be X.

Less books , Less binders       (Direct Proportion)

More days, Less binders         (Indirect proportion)

  $Books900:660Days12 : 10⋮⋮ 18:x$

Therefore, (900 * 12 * x) =  (660 * 10 * 18)  

=> x = 11