I'm looking at a textbook problem, which has the answer as well, but don't understand what's going on.
The problem: A part with lifetime X has a normal distribution with mean of 7 years and standard deviation of 1.2 years. If the warranty is for 5 years, what % will fail inside the warranty?
The answer is: P(X<=5) = P(Z<=(5-7)/1.2) = P(Z<=-1.67) = .5-.4525 = 0.0475
I follow everything until the .5-.4525 part. Where do these numbers come from?