You don't. Five of you (at last count) look at problems like this one and say "Gee, this looks like fun." And then you whip out your tools, pull some figures for mass and gravitational acceleration off the web (or off the top of your heads), crunch the numbers, and give me results.
Sadly, I'm ill-prepared to grade these results. I'm still trying to make sense of them. I think I owe the preparators at least that much effort. This is what I get for pretending to be hard sci-fi when in fact I'm a comic strip. (Please do not tell my wife I am a comic strip. She is under the assumption that I'm a human being, and I'd like to keep her deluded for a while longer).
For those of you who are bored by math, but love sci-fi and want a good laugh, check out the newest sci-fi strip here in the Keen Universe. It's called Gravity, and it is being created under the hand of a true comic-book professional, D.J. Coffman. It's got planet-eating monsters, robot babysitters, and space pirates! (pirates coming soon)