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.