A Company That Receives Most of Its Orders by Telephone

A company that receives most of its orders by telephone conducted a study to determine how long customers were willing to wait on hold before ordering a product.The length of waiting time was found to be a variable best approximated by an exponential distribution with a mean length of waiting time equal to 3 minutes (i.e.the mean number of calls answered in a minute is 1/3) .Find the waiting time at which only 10% of the customers will continue to hold.

A) 2.3 minutes
B) 3.3 minutes
C) 6.9 minutes
D) 13.8 minutes