Students used “5 cars and 5 bikes” as reference point. They calculated the total number of wheels as 30. Then they checked and adjusted the number of cars and bikes and calculated the combination of the number of wheels. If they found appropriate answer, they would circle them. From their process of problem solving, students seemed to take the number of cars as a sure variable. They understood that when the number of cars increased by 1 and the number of bikes decreased by 1, then the number of wheels will increase by 2.