Mars has two moons, Phobos and Deimos. Phobos orbits Mars at a distance of 9380 km from Mars's center, while Deimos orbits at 23,500 km from the center.
What is the ratio of the orbital period of Deimos to that of Phobos?

The ratio is  

Explanation:

From the question we are told that

   The  radius of Phobos orbit is  R_2 =  9380 km

    The radius  of Deimos orbit is  

Generally from Kepler's third law

Here M is the mass of Mars which is constant

        G is the gravitational  constant

So we see that

=>  

Here is the period of Deimos

and   is the period of  Phobos

So

=>    

=>    

inverse v=i*r

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