A brand of television has a lifetime that is normally distributed with a mean of 7 years and a standard deviation of 2.5 years. What is the probability that a randomly chosen TV will last more than 8 ...
The mean is 7 years, so there is .5 probability that it will last 7 years or more.
The standard deviation is 2.5, so one additional year is 1/2.5 = 0.4 S.D.
The probability that it will fail in that additional year is the probability table entry for 0.4 S.D. = .155 so the probability that it will last 8 years or more is .5 - .155 = .345