A rational expression is one polynomial divided by another, like 200/(x + 20) for the percent of plant food after x liters of water. Simplify by factoring and canceling, add with a common denominator, and rewrite a(x)/b(x) as q(x) + r(x)/b(x) by division. Solve a rational equation by clearing the denominators, then throw out any extraneous solution that makes a denominator zero.