(1) the diameter is 20 inches whats the radius? And whats the circumference?
(2) the diameter 11 inches whats the radius? And whats the circumference?
(3) the diameter 3.5 inches whats the radius? And whats the circumference?

(1) the diameter is 20 inches whats the radius? And whats the circumference?

Radius is half the diameter, 10 inches

Circumference is pi times the radius, 10π inches

I won't bother with the approximation, though this one is easy, 31.4159 inches.  I'll leave the rest of the calculator work to you if necessary.

(2) the diameter 11 inches whats the radius? And whats the circumference?

Radius half, 5.5 inches.   Circ: 5.5π inches

(3) the diameter 3.5 inches whats the radius? And whats the circumference?

Radius, 1.75 inches.  Circ: 1.75π inches

12/11c

or

12

c

11