12 cm.
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The radius of the circle is half the diameter of the circle. 30mm radius would mean 60mm diameter or 6cm.
Well, isn't that just a happy little question! To find the diameter of the circle, we simply need to double the radius. Since the radius is 30 mm, the diameter would be 60 mm. To convert that to centimeters, we just need to divide by 10, so the diameter of the circle is 6 cm.
The radius of a circle is half of its diameter
1/2 of the diameter
The diameter of a circle is twice its radius. Thus the radius of a circle is half its diameter. → a circle with a diameter of 15 cm has a radius of 15/2 cm = 7.5 cm