Let’s solve each problem step by step.
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Problem 1: Circle with diameter = 18 in
We are given the
diameter, but the formula for circumference is:
> C = 2πr
> OR since diameter d = 2r, we can also use:
>
C = πd
So if diameter = 18 inches, then:
C = π × 18 ≈ 3.14 × 18 =
56.52 inches
✔ Answer:
56.52 in
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Problem 2: Circle with radius = 5 in
Use C = 2πr
C = 2 × π × 5 = 10π ≈ 10 × 3.14 =
31.4 inches
✔ Answer:
31.4 in
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Problem 3: Circle with diameter = 4 cm
Again, use C = πd
C = π × 4 ≈ 3.14 × 4 =
12.56 cm
✔ Answer:
12.56 cm
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Problem 4: Circle with diameter = 39.8 mm
C = π × 39.8 ≈ 3.14 × 39.8
Let’s calculate that:
3.14 × 39.8
= 3.14 × (40 - 0.2)
= 3.14×40 - 3.14×0.2
= 125.6 - 0.628 =
124.972 mm
Rounded to two decimals:
124.97 mm
✔ Answer:
124.97 mm
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Problem 5: Radius = 2.5 cm → Find diameter
Diameter = 2 × radius
= 2 × 2.5 =
5 cm
✔ Answer:
5 cm
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Problem 6: Diameter = 15 cm → Find radius
Radius = diameter ÷ 2
= 15 ÷ 2 =
7.5 cm
✔ Answer:
7.5 cm
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Final Answer:
- First circle (18 in diameter):
56.52 in
- Second circle (5 in radius):
31.4 in
- Third circle (4 cm diameter):
12.56 cm
- Fourth circle (39.8 mm diameter):
124.97 mm
- Radius 2.5 cm → Diameter:
5 cm
- Diameter 15 cm → Radius:
7.5 cm
Parent Tip: Review the logic above to help your child master the concept of circumference worksheets.
