Discrete probability distributions

[Mdhiggenz]

Homework Statement

A certain manufacture advertises batteries that will run under a 75 amp discharge test for an average of 100 minutes, with standard deviation of 5 minutes.

a. find an interval that must contain at least 90% of the performance periods fr batteries of this type.

b. would you expect many batteries to die out in less than 80min why or why not?

I solved a. to be (85,115)

however I'm not quite sure what to do about B. I would guess and say that know due to the interval which is from 85 to 115 which would mean that the battery life generally last in that range however, I'm just speculating.


Thanks

Higgenz

Homework Equations

The Attempt at a Solution

 

[haruspex]

How many standard deviations would 80 min be from the average? Do you have any tables which tell you the percentage of a normal distribution that would be that far or further below the mean?