A circle has area 25π. What is its radius?

• A. 5
• B. 1
• C. 10
• D. 15

Answer: (A)

The area of a circle is found by π times the radius squared. So you set πr^2 equal to 25π. Divide both sides by π to get r^2 = 25, and since a radius cannot be negative, r = 5. This radius indeed gives area π(5)^2 = 25π, matching the given value. Other radii would produce different areas—for example, r = 1 gives π, r = 10 gives 100π, and r = 15 gives 225π—so they don’t fit.