7.7 An engineer is studying the length of time his company's rechargeable batteries will work before needing to be recharged. He tests a random sample of 100 batteries and finds the average time that these batteries hold a charge is 120 hours with standard deviation of 12 h. Assume that the data exhibit only moderate skewness and find a 95% one-sided lower confidence bound for the true mean length of battery life, and give an interpretation of the bound