Normally distributed random variable and probability

Homework Statement

The top-selling Red and Voss tire is rated 60000 miles, which means nothing. In fact, the distance the tires can run until wear-out is a normally distributed random variable with a mean of 70000 miles and a standard deviation of 5000 miles.

A: What is the probability that the tire wears out before 60000 miles?
B: What is the probability that a tire lasts more than 79000 miles?

z = (Y-μ)/σ

The Attempt at a Solution

I plugged the values into the above equation and got z=-2. Looking at the chart in my book, Pr(z=2)=0.0228 and I figured this should work since z is 2 standard deviations from the mean no matter whether it's positive or negative. When I submitted 0.0228 though, the answer was incorrect.

Ray Vickson
Homework Helper
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Homework Statement

The top-selling Red and Voss tire is rated 60000 miles, which means nothing. In fact, the distance the tires can run until wear-out is a normally distributed random variable with a mean of 70000 miles and a standard deviation of 5000 miles.

A: What is the probability that the tire wears out before 60000 miles?
B: What is the probability that a tire lasts more than 79000 miles?

z = (Y-μ)/σ

The Attempt at a Solution

I plugged the values into the above equation and got z=-2. Looking at the chart in my book, Pr(z=2)=0.0228 and I figured this should work since z is 2 standard deviations from the mean no matter whether it's positive or negative. When I submitted 0.0228 though, the answer was incorrect.

Your answer is correct. Who told you it is incorrect?

RGV

It's an online homework assignment. I talked to my teacher and he said the site was having issues with the rounding. Thanks for the help!