A special industrial battery must have a life of at least 500 hours. A hypothesis test is to be conducted with a 0.04 level of significance. If the batteries from a particular production run have an actual mean use life of 485 hours, the production manager wants a sampling procedure that only 10% of the time would show erroneously that the batch is acceptable. What sample size is recommended for the hypothesis test? Use 20 hours as an estimate of the population standard deviation.