Io and Callisto are satellites of Jupiter. Their periods are $1.77 d$ and $16.7 d$. What is the ratio of their distances from Jupiter?
I assume that I have to use Kepler's third law of planetary motion to find the ratio. I plugged the periods into the equation : $\dfrac{T_1^2}{r_1^3}=\dfrac{T_2^2}{r_2^3}$ and got the ratio of $89.02$. I am not sure of the units but is this the correct answer?
I think that you have to take the cube root of your answer. This is a frequent error, but one that can be easily corrected. However, you do not have to worry about the units, because a ratio of equal units will cancel, and will be dimensionless