Presumably you mean "radius"... 70 pi sq mm
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The circumference of a circle with a diameter of 13 mm is: 40.84 mm
The circumference of a circle that is 16 mm in diameter is 50.27 mm
A circle with a radius of 15.5 mm has a circumference of 97.39 mm.
Well, darling, the circumference of a circle is calculated using the formula C = 2πr, where r is the radius. Plug in the radius of 4 mm, and you get a circumference of 8π mm, or approximately 25.13 mm. So, there you have it, sugar, the circumference of a circle with a radius of 4 mm.
I'm wondering what the original, or book, question actually asks, but in this case, a it is a circle larger in radius, diameter and circumference by the ratio 4:3; and in area by the ratio 16:9.