1. The problem statement, all variables and given/known data "If Ganymede, one of Jupiter's moons, has a pariod of 7.15 days, how many uits are there in it's orbital radius? Use the information given in Example Problem 1." Example Problem: "Galileo measured the orbital sizes of Jupiter's moons using the diameter of Jupiter as a unit of measure. He found that Io, the closest moon to Jupiter, had a period of 1.8 days and was 4.2 units from the center of Jupiter. Callisto, the fourth moon from Jupiter, had a period of 16.7 days. Using the same units that Galileo used, predict Callisto's distance from Jupiter." 2. Relevant equations (Ta/Tb)^2=(Ra/Rb)^3 3. The attempt at a solution My solution to the example problem: Ta = 1.8 Tb = 16.7 Ra = 4.2 (1.8/16.7)^2 = (4.2/Rb)^3 .0116 = 74.088(Rb^3) (Dividing 1.8 by 16.2 and squaring, and cubing 4.2 and Rb) .2264Rb=4.2 (Multiplying by Rb^3 and then taking the cubed root of everything) Rb=18.5512 (Dividing by .2264) I considered that since the book gave an answer of 19, that I was close enough, considering the book's want to round everything. For the problem I'm having difficulty with, I did this: Ta = 1.8 Tb = 7.15 Ra = 4.2 (1.8/7.15)^2 = (4.2/Rb)^3 .2517^2=4.2^3/Rb^3 (1.8 divided by 7.15) .0634=4.2^3/Rb^3 (Squaring previous answer) .0634(Rb^3)=4.2^3 (Multiplying by Rb^3) .3987Rb=4.2 (Taking the cubed root of both sides) Rb=10.5342 (Dividing by .3987) The book, however, lists an answer of exactly 4. I can't figure out where I messed up.