You and your roommate are moving to a city \(320 \mathrm{mi}\) away. Your roommate drives a rental truck at a constant \(60 \mathrm{mi} / \mathrm{h}\), and you drive your car at \(70 \mathrm{mi} / \mathrm{h}\). The two of you begin the trip at the same instant. An hour after leaving, you decide to take a short break at a rest stop. If you are planning to arrive at your destination a half hour before your roommate gets there, how long can you stay at the rest stop before resuming your drive?