We have to find the number of binders. Let the number of binders be x.

Direct Proportion:Less Books ($\downarrow$),Less binders($\downarrow$)

Indirect Proportion:More days ($\uparrow$),Less binders ($\downarrow$)

18 : x :: $\begin{cases}900 &: 600---(books) \\12 &: 10---(days) \end{cases}$

x × 900 × 12 = 18 × 600 × 10

x = $\dfrac{18\times 600 \times 10}{900 \times 12}$ = 10