# Whats the probability that a certain amount of batteries life longer than 3.25 hours?

The mean and standard deviation of the lifetime of a battery in a portable computer are 3.5 and 1.0 hours respectively.

So, what is the probability that the mean lifetime of 25 batteries exceeds 3.25 hours? Also, what is the probability that the mean lifetime of 100 batteries exceeds 3.25 hours?

How do i go about this? I just assumed it was normally distributed. So do i find $$z=\frac{3.25-3.5}{\sqrt{25}},$$ and then use a stat table?

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Since the mean and standard deviation of the lifetime of a battery in a portable computer are 3.5 and 1.0 hours respectively, the mean and standard deviation of the lifetime of 25 batteries in a portable computer are 3.5 and 1.0/5 hours respectively. Thus, z = (3.25 - 3.5)/0.2 .

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