Problem: Customers send e-mails to a help desk of an online retailer every 7 minutes, on average, and the standard deviation of the interarrival time is also 7 minutes. The online retailer has only one employee answer emails. It takes on average 5 minutes to write a response e-mail. The standard deviation of service times is also 5 minutes. The service duration has an exponential distribution and the interarrival time also has an exponential distribution.
How may e-mails would there be, on average, that have been submitted to the online retailer but not yet opened by the employee? (Provide your answer as a number with decimals; do not use fractions)