Solving Applied Problems: Two Equations?

You have two equations here. If x is the number of 30 second commercials and y the number of 60 second commercials, x + y is 12 and 30*x + 60*y = 10*60, or 600. So I'd recommend substitution. Y = 12 - x. 30 * x + 720 - 60*x = 600. 30 * x = 120. X = 4, and therefore y = 8. You can check this: 4 * 30 + 8 * 60 = 600 seconds, or 10 minutes.