After many years of product car batteries testing, a company knows that the average life of a battery is normally distributed, with a mean of 47 months and a standard deviation of 10 months. If they...

After many years of product car batteries testing, a company knows that the average life of a battery is normally distributed, with a mean of 47 months and a standard deviation of 10 months.

If they do not want to make refunds for more than 17% of their batteries under the full refund guarantee policy, for how long should the company guarantee the batteries (to the nearest month)