Solution for QID #28284: A company manufactures microwaves. The microwaves are known | StudyHelpMe

A company manufactures microwaves. The microwaves are known from past observations to have an average life of 27 months and a standard deviation of microwave lifetimes of 5 months. The distribution of lifetimes of the microwaves is normal. If the company does not want to make refunds on more than 12% of the microwaves it makes, how long should the guarantee period be (to the nearest month)? (manipulation of the formula for z only: z=(x-μ)/σ)