Question

I need help on B and C. Not sure how I can solve those. Can someone please help me out here? Thank you for your help!

Answer 1

Nice job with part A on getting those correct answers.

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Part B

• mu = mean = 43
• sigma = standard deviation = 3

Let's calculate the z score for the raw score x = 37

z = (x - mu)/sigma

z = (37 - 43)/3

z = -6/3

z = -2

A negative z score tells us we're below the mean. Specifically, we are exactly 2 standard deviations under the mean.

If you repeat those steps for x = 46 (not changing mu or sigma), then you should get z = 1. So we're now 1 standard deviation above the mean.

Ultimately, we want to know the area under the standard normal curve (aka z curve) from z = -2 to z = 1.

Refer to the chart below which is a breakdown of the Empirical Rule. The pink areas are within 1 standard deviation of the mean. We have two regions taking up roughly 34% of the full area under the curve, so they combine to 34+34 = 68%

Then we add on another 13.5% which is the blue area on the left (between z = -2 and z = -1)

So 13.5+68 = 81.5% of the area under the curve is between z = -2 and z = 1.

Answer: 81.5

Note: the percent sign is already taken care of for us, so there's no need to type it in. But the answer above of course means 81.5% and that answer is approximate.

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Part C

Let's find the z score for x = 34. The values of mu and sigma are the same as before.

z = (x - mu)/sigma

z = (34-43)/3

z = -9/3

z = -3

So computing P(X > 34) is the same as P(Z > -3)

Notice how adding up all of the values in the chart mentioned gets us:

2.35+13.5+34+34+13.5+2.35 = 99.7

which means 99.7% of the distribution is within 3 standard deviations of the mean. The remaining 100% - 99.7% = 0.3% is the combined area of both tails. So each tail is (0.3%)/2 = 0.15%

In short, 0.15% of the area under the curve is to the left of z = -3

Therefore, 100% - 0.15% = 99.85% of the widgets are going to have z values larger than -3, which in turn means they have values larger than 34.

Answer: 99.85

Again you don't have to worry about the percent sign. Like before, the answer is approximate.