Intro to Exponential Decay

Imagine you're waiting in line for ice cream and you're the very last person in line. You notice that the number of people in line decreases by $30\%$ every minute. If there were initially $20$ people when you got in line and no one came after you in line, how long will you wait before it's your turn to order? In other words, how many minutes will pass before only $1$ person (you) remains?