What is the circumference of a circle with radius 5?

• A. 25π
• B. 5π
• C. 10π
• D. 20π

Answer: (C)

The key idea is that the circumference of a circle is found using C = 2πr, meaning you multiply the radius by 2 and by π. With a radius of 5, that’s 2 × 5 × π = 10π.

You can also use C = πd, where the diameter is twice the radius. The diameter here is 10, so C = π × 10 = 10π. Both paths lead to the same result.

If you imagine other possible circumferences, they would require different radii: for example, 20π would correspond to a radius of 10, 25π to a radius of 12.5, and 5π to a radius of 2.5. So the correct circumference for this circle is 10π.