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The question below is simple, however, I got the question wrong despite following a basic formula I have been using for a while now.

Here's the question:

For applicants who actually entered medical school, the mean score was 10.4 and the standard deviation was 1.8. What percent of those who entered medical school had scores between 9 and 11?

My answer for the question is : 14.012%

Here is what I did, I subtracted 9 from 10.4 then divided by 1.8 to get 0.3. I also subtracted 11 from 10.4 then divided by 1.8 to get 0.7.

In my Statistics class, we use this program called, Minitab. I used the Normal Distribution cumulative calculator. I got 0.758036 - 0.617911 = 0.140125, then I multiplied my final result by 100 to get 14.0125%

Why is it incorrect? I literally follow the same format when coming across a question with the keyword "between."

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    $\begingroup$ "I subtracted 9 from 10.4 then divided by 1.8 to get 0.3. I also subtracted 11 from 10.4 then divided by 1.8 to get 0.7." -- I'd focus on what you did here. Note that since 9 and 11 are on opposite sides on the mean, their z-scores cannot have the same sign, yet yours do. $\endgroup$
    – Glen_b
    Commented Dec 3, 2018 at 5:11

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You reversed the formula on accident. The actual formula is:

Z = (X- mu)/sigma

You used Z= (mu-X)/sigma.

So it should be (9-10.4)/1.8.

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