The ratio of Don\textquotesingle s stickers to Ben\textquotesingle s stickers was $2 : 3$. After Don\textquotesingle s bought another $10$ stickers and Ben lost $5$ stickers, the ratio became $6 : 7$. Find the number of stickers Don had at first. 
Overseas Competition->Knowledge Point->Calculation Modules->Ratios and Proportions