9. The diagram shows five equal semicircles and the lengths of some line segments. What is the radius of the semicircles?
A 12
B 16
C 18
D 22
E 28
Solution
C

Let the radius of each semicircle be $r$. From the diagram we can see that $2 r+12+2 r+12+2 r=$ $22+2 r+16+2 r+22$ and hence $6 r+24=4 r+60$. Therefore $2 r+24=60$ and so $2 r=36$ and hence $r=18$.