stats
posted by Carlos .
The life of tires manufactured by Omega Tires Inc is believed to be normally distributed with a mean of 110,000 km and a standard deviation of 5,000 km. A random sample
of 200 tires is tested . What is the problability of the mean life of this sample being between 109,000 and 111,000 km?

Z = (scoremean)/SEm
SEm = SD/√n
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion related to the Z scores.