Memoryless waiting time at a counter

At a single-staff checkout counter of a supermarket store, the time taken in minutes to complete the service of a customer is a random variable having probability density function given by $f(x) = \begin{cases} \frac{1}{10} e^{-x/10} & \text{if } x \ge 0 \ 0 & \text{otherwise.} \end{cases}$ When you arrive at the counter, you observe that there is already one person in service. You are also informed that the person has been in service for 5 minutes. Assuming that the service times of different customers are independent of each other, the probability that your total waiting time (which is the sum of your waiting time in queue and service time) is more than 15 minutes equals