Legs for a wooden dining room table are produced by a computer numerically controlled (CNC) lathe. The cutting blade lasts a mean time of 550 h with a standard deviation of 20 h. To avoid cutting errors, the shop manager would like to keep the probability of a failure to less than 0.002. How many hours should the blade be used before replacement? Explain.
I know this answer is 492.4 hours, but how do you get it?