Crazy Keyboardz Inc., sells an expensive keyboard package, including a pre-built custom keyboard. Because the keyboard is so expensive, the company wants to advertise an impressive guarantee for the sound level (the keyboard promises to be very quiet) of the keyboard for 20 months. The guarantee policy will refund the full purchase price if the keyboard fails (becomes noisy) during the guarantee period. The market research department has done tests that show that the mean life for the keyboard is 20 months, with standard deviation of 3 months. After 20 months, most keyboards become loud and can break. The keyboard life is normally distributed. How long can the guarantee period be if management does not want to refund the purchase price on more than 5% of the Crazy Keyboardz packages? (10)