[math] \newcommand{\NA}{{\rm NA}} \newcommand{\mat}[1]{{\bf#1}} \newcommand{\exref}[1]{\ref{##1}} \newcommand{\secstoprocess}{\all} \newcommand{\NA}{{\rm NA}} \newcommand{\mathds}{\mathbb}[/math]

Assume that the service time for a customer at a bank is

exponentially distributed with mean service time 2 minutes. Let [math]X[/math] be the total service time for 10 customers. Estimate the probability that [math]X \gt 22[/math] minutes.