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Three concentric circles have diameters in arithmetic progression. The smallest circumference is 8π cm and the largest is 18π cm. What is the middle circumference?

1. 11 π cm
2. 12 π cm
3. 13 π cm
4. 14 π cm

Correct answer: 13 π cm

Solution

Since circumference is proportional to diameter, the three circumferences are also in arithmetic progression. Therefore, the middle circumference is the average of 8π and 18π, which is 13π cm.

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