It Is Estimated That the Time Between Arrivals of Visitors $\mathrm { e } ^ { \mathrm { a } / \mathrm { b } }$

Short Answer

It is estimated that the time between arrivals of visitors to a public library is an exponential random variable with expected value of 13 minutes. Find the probability that 30 minutes elapses without any arrivals. Enter your answer as just $\mathrm { e } ^ { \mathrm { a } / \mathrm { b } }$ , where $\frac { a } { b }$ is a reduced fraction.