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The mean ACT score is given as 51.1.

The standard deviation of ACT scores is given as 5.1.

Thez-scoreof a value from a sample is computed using the following expression:

$z=\frac{x-\stackrel{炉}{x}}{s}$

A value whose score is less than or equal to -2 is considered to be significantly low.

A value whose score is greater than or equal to 2 is considered to be significantly high.

The following calculation is done to compute the values of scores that are significantly low:

$$\begin{gathered} z猢�-2 \\ \frac{x-\stackrel{炉}{x}}{s}猢�-2 \\ \frac{x-21.1}{5.1}猢�-2 \\ x猢�10.9 \end{gathered}$$

Therefore, ACT scores equal to or below 10.9 aresignificantly low.

The following calculation is done to compute the values of scores that are significantly high:

$$\begin{gathered} Z猢�2 \\ \frac{x-\stackrel{炉}{x}}{s}猢�2 \\ \frac{x-21.1}{5.1}猢�2 \\ x猢�31.3 \end{gathered}$$

Therefore, ACT scores equal to or above 31.3 aresignificantly high.