The local Beep Boop store sells widgets and gizmos. If you subtract the cost of 3 gizmos from 7 widgets you would spend 28 Boop Bucks. Also, you would spend 16 Boop Bucks if you bought 4 widgets and 2 gizmos. Which of the two items, widgets or gizmos, does the Beep Boop store give away for free? How do you know? How much does the other item cost?