The durations of phone calls taken by the receptionist at an office are like draws made at random with replacement from a list that has an average of 8.5 minutes (that's 8 minutes and 30 seconds) and an SD of 3 minutes. Approximately what is the chance that the total duration of the next 100 calls is more than 15 hours?

Question

agent0smith · Answer

First convert everything into seconds.

The mean is 8.5 minutes for one call, so it's 100 times that amount for 100 calls.

The SD is 3 minutes, the variance is the SD squared. 

Multiply the variance by 100, then take the square root of it for the SD for 100 calls.

$$\Large z = \frac{ x - mean }{ SD }$$
x is the time for 100 calls. The mean is the mean for 100 calls, SD is the SD for 100 calls. You'll use the z score to find the probability.

anonymous · Answer

agensmith could you clarify Your approach more ?