Call the price of large is x, the price of small is y.

We have : \(\left\{{}\begin{matrix}7x+4y=100\\5x+6y+5=100\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}7x+4y=100\\5x+6y=95\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}x=10\\y=7,5\end{matrix}\right.\)

So a small exercise book costs 7,5$.

Let L and S be the price of a large and a small one respectively.We have :

\(\left\{{}\begin{matrix}7L+4S=100\\5L+6S+5=100\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}35L+20S=500\\35L+42S=665\end{matrix}\right.\)

\(\Rightarrow22S=165\Rightarrow S=7,5\)

So,a small exercise book costs 7,5