Intro to Exponential Decay

You're driving a car, going $50$ miles per hour ($180,000$ miles per second), when you notice that the speed limit is reduced to $25$ mph ($90,000$ miles per second) as you enter a work zone. You hit the brakes and your speed begins to decrease exponentially. After $10$ seconds, you've slowed to the speed limit of $25$ mph. What is the rate of speed decrease in $\textbf{miles per second}$?