Long Life Battery Company makes car batteries. After years of product testing, the company knows their batteries follow a normal distribution with a mean lifespan of 45 months and a standard deviation of 7 months. The company guarantees that batteries with a short lifespan will be replaced free of charge. If Long Life does not want to make refunds for more than 10% of its batteries under the full-refund guarantee policy, for how long should the company guarantee the batteries?

What equation should be used for this problem z= x- µ σ or  p(x)=(nCx)px 1- p ( )n-x  or x = µ + z σ   z= x - µ σ n e) None of the above are correct.