Question 1122212
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You can place O at 0 on a number line; then B will be at 9 and A will be at 12.<br>
Having drawn that picture, you should be able to answer the question.<br>
We can't answer it for you, because you ask for "the distance between the midpoints of the segments", but you don't specify which two of the three segments (OA, OB, and AB) you are talking about.