This puts a little algebra into a more interesting context than figuring out the time when a train leaving Chicago going 60 miles an hour will meet up with a train leaving Kansas City traveling 45 miles per hour and how far they will be from Chicago. The suggestion is to calculate the percentage of rock in Callisto, which has a density of 1.8 grams per cubic centimeter. The constants 3.5 and 0.9 are simply the densities of rock and ice, respectively. The problem is that we have one equation with two unknowns. This dilemma can be solved by a substitution.
Students can then check to see if it agrees with their result from the graph.
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