Let P(x) = probability of seeing at least one car in x minutes: . The probability of seeing at least one car in 30 minutes is one minus the probability of not seeing a car in 30 minutes, or . Next, . In other words, to not see a car in 30 minutes is the same as not seeing a car in 10 minutes three times in a row. Hence:

From this you solve for P(10), and get