$$M$$ is the set of all odd integers between $$A$$ and $$B$$. If $$A$$ and $$B$$ are even integers such that $$A \lt B$$, what is the average (arithmetic mean) of $$M$$?

Correct.
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When you plug in $$A=2$$ and $$B=8$$ into this answer choice, you get
>$$\frac{A+B}{2} = \frac{2+8}{2} = \frac{10}{2} = 5$$
This is the right answer choice because all other answer choices are eliminated for the same Plug-Ins.

Incorrect.
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$$\frac{A-B}{2}$$

$$\frac{A-B}{2}+1$$

$$\frac{A+B-1}{2}$$

$$\frac{A+B}{2}$$

$$\frac{A+B+1}{2}$$