Objects can be represented in a reduced or augmented size by using scaling which consists of multiplying or dividing by a constant factor. We use scaling when representing geographic locations on a map.
On a scale drawing, a distance of 5 meters = 500 cm is represented by 4 centimeters. The scale factor is usually represented as a ratio of the smallest to the biggest magnitude:
To get the length of a spoke, you would divide 42 by pi (giving you diameter), then divide that number by 2 (to get the radius). (42/pi)/2=6.68 This is your answer.