Andy drove $70$ miles in $90$ minutes. His average speed during the first $30$ minutes was $x$ miles per hour, and his average speed during the scond $30$ minutes was $(x-20)$ mph. His average speed during the last $30$ minutes was $(x+10)$ miles per hour. What is the value of $x$? 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Linear Equations with one Variable