Once you know how long each moon takes to orbit Jupiter, you can work out a few things:
First, you can measure the orbits, since you know how big Jupiter's is, and how far away it is, etc. You can work out the semimajor axis of the orbits as a fraction of Jupiter's real size, then you just multiply by Jupiter's real size to find roughly the semimajor axis of the orbits.
Now using the generalised form is here:
http://en.wikipedia.org/wiki/Kepler%...tion#Third_law, just use Jupiter's mass for M, and the semimajor axis of the orbits as a, and you should be able to work out what m is, as a fraction of M. So you would just really need to set M = 1, then m would be some very small number.
Then, you can look up what mass of Jupiter /really/ is, and work out from that how close you are :-)
The really cool part, is where you compare your results against "real" results, and explain *why* yours were different.