How many bikes and tricycles does Rick's bike shop have if the inventory shows 117 wheels and 104 pedals?

1. Each bike has 2 pedals and 2 wheels;

2. Each tricycle has 2 pedals and 3 wheels;

3. Let x be the number of bikes and, y the number of tricycles;

4. 2x + 3y = 117 and 2x + 2y = 104 = > (2x + 3y) - (2x + 2y) = 117 - 104 = > y = 13 = > 2 * x + 2 * 13 = 104 = > 2 * x = 104 - 26 = > 2 * x = 78 = > x = 78 / 2 = > x = 39.