A circle has a diameter 16 and an area of 64?

Hold it right there. That's not possible. Something's gotta give.

The area of a circle = pi R2 = pi (half the diameter)2

If the circle has a diameter of 16, then its area is 201.1(rounded).

If it has an area of 64, then its diameter is 9.03(rounded).

If it has a diameter of 16 and an area of 64, then it's not a circle.