The original question should be: Future Electronics makes compact disc players. Its research department found that the life of the laser beam device is normally distributed, with mean 4820 hours and standard deviation 420 hours. Future Electronics wants to place a guarantee on the players so that no more than 10% fail during the guarantee period. Because the laser pickup is the part most likely to wear out first, the guarantee period will be based on the life of the laser beam device. How many playing hours should the guarantee cover?