The life in hours of a thermocouple employed in furnace is known to be approximately normally distributed, with standard deviation ? = 20 hours. A random sample of 15 thermocouples resulted in the following data: 553, 552, 567, 579, 550, 541, 537, 553, 552, 546, 538, 553, 581, 539, 529. We wanted to be 95% confident that the error in estimating the mean life is less than 5 hours. What sample size should we use? Round your asnwer up to the nearest whole number. The answer must be exact.