A question about normal distribution and how to get a percentage

Question

The life of a fully-charged cell phone battery is normally distributed with a mean of 14 hours with a standard deviation of 1 hour. What is the probability that a battery lasts at least 13 hours?

Answer 1

Answer 2

x = number of hours

want to find probability (P) x >= 13

x is N(14,1) transform to N(0,1) using z = (x - mean) / standard deviation so can look up probability using standard normal probability table.

P(x >= 13) = P( z > (13 - 14)/1) = P(z > -1) = 1 - P(z < -1) = 1 - 0.1587 = 0.8413

To convert that to percentage, multiply 100, to get 84.13%