Intro to Exponential Decay

You're driving a car, going $60$ miles per hour ($216,000$ miles per second), when you notice that the speed limit is reduced to $35$ mph ($126,000$ miles per second). You hit the brakes and your speed begins to decrease exponentially by $20\%$ every second. How many $\textbf{seconds}$ will pass before you have slowed down to the speed limit?