Question

The shelf life of a particular dairy product is normally distributed with a mean of 12 days and a standard deviation of 3 days. About what percent of the products last between 9 and 12 days?

Answer 1

$$P(9\le X\le 12)=P(Z\le\dfrac{12-12}{3})$$

$$-P(Z<\dfrac{9-12}{3})=P(Z\le 0)-P(Z<-1)$$

$$=0.5-0.1586553=0.341345$$