statistics
posted by jericho on .
a manufacturer of bicycle tire has developed a new system which he claims an average life span of 5 years. with the standard deviation of 1.2 years a dealer product claims that the average life span of 150 samples is only 3.5 years. test the difference of population and sample means at 5% level of significance. what is the answer?

Do you really mean samples? If so, what is the n for each sample? If you really mean n, use information below.
Z = (mean1  mean2)/standard error (SE) of difference between means
SEdiff = √(SEmean1^2 + SEmean2^2)
SEm = SD/√n
If only one SD is provided, you can use just that to determine SEdiff.
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion related to the Z score.