How Long Does Moon E Take to Orbit Planet?

[Want to learn]

Homework Statement

Two moons orbit a planet in nearly circular orbits. Moon D has orbital radius r, and moon E has orbital radius 4r. Moon D takes 20 days to complete one orbit. How long does it take for moon E to complete one orbit

Homework Equations

None - I think

The Attempt at a Solution

I am hoping that this problem is not complicated. Since it takes 20 days for moon D with radius r, it would take 80 days for moon E with radius 4r?

Is my reasoning correct ??

 

[D H]

No, it isn't. What do Kepler's laws have to say about this problem?

 

[Want to learn]

Is it just 20 days the same as moon D?

A line drawn from the sun to any planet sweeps out equal areas in equal time intervals.

Does the size - radius matter then?

 

[Want to learn]

Ok the above doesn't make sense.

Then I don't know, really.

 

[willem2]

It's not just 20 days.

Keplers second law is only valid for the same planet or moon (and it's trivial for a circular orbit)
Try the third law.