The mean (average) equals 20, and the standard deviation equals 3. For standard deviation, remember to consider numbers above and
below the mean.
One standard deviation includes all numbers that fall into 17 to 23. This is determined by adding and subtracting 3 from 20. In other words, 20 + 3 = 23, and 20 - 3 = 17. We can eliminate C.
Two standard deviations identifies the set of numbers that fall into 14 ( 20 - 3 - 3) to 26 (20 + 3 + 3). We can eliminate D.
2.5 standard deviations results in 12.5 to 27.5. We can eliminate B and E. Hence, A.
Lagomez, of course, has already shown us a faster way.
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