Answer:
She made a mistake in Step 7.
Step-by-step explanation:
Step 1 through 6 are correct. Only from Step 6 to Step 7 the problem comes in: both sides of the equation are multiplied by x, however the x on the left side, by error, becomes 1. The correct state in Step 7 should look like this:
A suitable probability calculator can tell you the probability is about 6.7%.
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The standard deviation of the distribution of sample means is the population standard deviation (6) divided by the square root of the sample size (√144=12). Thus, your threshold is about (19.25-20)/(6/12) = -1.5 standard deviations from the mean.
