$\mathrm{Mary}$ read several books each time. On the first day $\mathrm{7}\mathrm{pages}$, on the second day $\mathrm{8}\mathrm{pages}$ of the book were read. If the number of pages read on the first day, divided by the sum of pages read on the second and third days equals $\frac{28}{68}$, how many pages did $\mathrm{Mary}$ read on the third day? 

$\mathrm{Mary}$ read