Acatalog company that receives the majority 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 2.8 minutes (i. e. the mean number of calls answered in a minute is ). what proportion of callers is put on hold longer than 2.8 minutes?