Suppose that the lifetime of a farm tractor follows a normal distribution with a mean of eight years and a standard deviation of 1.7 years. If the manufacturer is willing to replace only 5% of the tractors that fail within the warranty period, how long should the warranty period be?

Answer 1

http://davidmlane.com/hyperstat/z_table.html

Answer 2

Answer and solution pleaseee

Answer 3

Hey, just try it
Value from area circle
area = .95 (which is 1 - .05)
mean = 8
sd = 1.7
then black area (.95 still working) is below 10.797 years