What is the area of a circle with a diameter of 20 inches if π= 3.1415?

The story behind the answer

Diameter 20 inches gives radius r = 10 inches. The area of a circle is A = πr², so with π = 3.1415: A = 3.1415 × 10² = 3.1415 × 100 = 314.15 square inches, matching the correct option.

Why this works: π is the constant ratio of a circle’s circumference to its diameter. If you slice a circle into many thin wedges and rearrange them, they approximate a rectangle of width r and length πr, giving area πr². Memory tip: “Pi r squared” (even though pie is round!)—and always use the radius, not the diameter, in the area formula.