It might help if you draw a pictures. Each circle across measures
5 inches (radius of 2.5 means diameter of 5) The three circles in
the middle all contribute to the
measurement 5 * 3 = 15. You have 2 halves of a diameter with
the first circle and the last circle, so that has a total of
2.5 + 2.5 = 5. Added to the other total, the
measurement is 15 + 5 = 20 inches (C).