1. The problem statement, all variables and given/known data The circumference of a ball is 29.6 inches. Given that the radius of Earth is about 6400 km, how many balls would it take to circle around the equator with the balls touching one another? 2. Relevant equations (d)(pi) = circumference 1 inch = 0.0254 meters 1 km = 1000 meters 3. The attempt at a solution Ball diameter = ((29.6)(0.0254))/3.14 = 0.239 m Earth circumference = (6400)(2)(3.14)(1000) = 40,192,000 m # of balls circumnavigating the equator = 40,192,000/0.239 = 168,167,364 In the back of the book it says 26,700,000 balls (it only uses 3 significant digits) and I can get that if I divide the radius of the earth by the ball diameter: ((6400)(1000))/0.239 = 26,700,000 I don't see why they are using the radius when it says around the earth, not halfway through it. Am I right here? Didn't know if there was some sort of magic I was missing.