What is the probability a battery lasts for more than 30 hours on one charge?

Apple observes that the battery in the iPhone 5 lasts for an average of 26 hours under normal usage. Apple believes that battery life per charge is normally distributed, with a standard deviation of 5 hours.

What is the probability a battery lasts for more than 30 hours on one charge?

please explain

It depends. Giving you a probability would be all well and good but completely useless. People buy iPhones to look cool, because they love apple or because its a business phone (in most cases). The chances of the battery lasting that long on standby are quite high, but with normal to heavy usage it will drain within maybe 12hrs. For some people, it might be ok if they charged it once every 2 days but as an iPhone user i charge it every night without fail. So to conclude it is impossible to give an accurate probability although i can safely say it will last 30 hrs on standby

Μ = 26
σ = 5
standardize x to z = (x - μ) / σ
P(x > 30) = P( z > (30-26) / 5)
= P(z > 0.8) = 0.2119
(From Normal probability table)