Answer: 10π + 20, or 51.4159, cm
Explanation:
The circumference of a circle is calculated using πd. Our d, or diameter, is 20, so the circumference of a full circle would be 20π. However, we only have half of a circle, so that number is halved, resulting in 10π. Now, we just have to add on the cross section, which is equal to the diameter, giving us 10π + 20, or 51.4159, cm.