Question: the length of life of a certain type of washer is approximately normal distributes with a mean=3.3 years and standard deviation 1.3 years. If this washer is guaranteed for 1 yr, what fraction of the sales will require replacements.  Edit

Answer: Given X~Normal(mean=3.3, s=1.3) So the probability is P(X>1)=P((X-mean)/s >(1-3.3)/1.3) =P(Z>-1.77) =0.9616 (check standard normal table)  Edit