Leonard drinks coffee that contains 100 mg of caffeine. Over the course of the day, every hour, one-half of the caffeine in his system leaves the body and one-half stays. Let x = the number of hours and y = the amount of caffeine in Leonard's system. If he finishes his coffee at 9:00 am, how many mg of caffeine are left in his system at 3:00 pm? Write the equation using the variables above that represents this situation and solve the problem, showing the calculation you did to get your answer.