Suppose that the talk time on the Apple iPhone is approximat

Suppose that the talk time on the Apple iPhone is approximately normally distributed with mean 9 hours and standard deviation of 11.5 hours.
a)   What proportion of the time will a fully charged iPhone last at least 6 hours?
b)   What is the probability a fully charged iPhone will last less than 5 hours?
c)   What talk time would represent the cut off for the top 5% of all talk times
d)   Would it be unusual for the phone to last more than 11.5 hours? Why?

Solution

a) P(x>6)=0.6029.

Use NORMDIST function and subtract with 1.

b) P(x<5)=0.3639. Use NORMDIST function.

c) Uisng NORMSINV(1-0.05)=1.645

1.645=x-9/11.5

solving for x

x=27.9 approximately 28.

d) P(x>11.5)=0.4139. So 41.39% of the time the phone lasts more than 11.5 hours. So it is not unusual.