Probability, Expected Value, Standard Deviation

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I did not understand part c. How should I calculate it? Can someone please show it?

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I did not understand part c.

after some calculations you get that

$$E(X)=7$$

$$\sigma(X)\approx 2.65$$

They request you to calculate

$$P(7-1.5\cdot2.65\leq X\leq 7-1.5\cdot2.65)=$$

$$=P(3<X<11)=1-2[P(X=2)+P(X=3)]=1-2\left[\frac{1}{36}+\frac{2}{36}\right]=\frac{30}{36}$$