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Answer:
 a.  d = C/π
 b.  d = 8/π inches ≈ 2.546 inches
 c.  r = 61/(2π) inches ≈ 9.708 inches
Step-by-step explanation:
a. The formula for circumference is ...
 C = πd
Dividing by π gives ...
 d = C/π
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b. For a circumference of 8 inches, the diameter is ...
 d = (8 in)/π = (8/π) in ≈ 2.546 in
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c. The radius is 1/2 the diameter. So, the radius of a circle is ...
 r = d/2 = (C/π)/2 = C/(2π)
For a circumference of 61 inches, the radius is ...
 r = (61 in)/(2π) = 61/(2π) in ≈ 9.708 in
