Summarize:

If the problem tells you the diameter of the circle, it's easy to find the radius. If you are working with an actual circle, measure the diameter by placing a ruler so its edge passes straight through the circle's center, touching the circle on both sides.  If you're not sure where the circle center is, put the ruler down across your best guess. Hold the zero mark of the ruler steady against the circle, and slowly move the other end back and forth around the circle's edge. The highest measurement you can find is the diameter. For example, you might have a circle with a diameter of 4 centimeters. A circle's radius is always half the length of its diameter.   For example, if the diameter is 4 cm, the radius equals 4 cm ÷ 2 = 2 cm. In math formulas, the radius is r and the diameter is d. You might see this step in your textbook as r=d2{\displaystyle r={\frac {d}{2}}}.
Check the problem for a diameter. Divide the diameter by two.