Let x be a number of skateboards and y be the number of bicycles in Eddie's repair shop.
Eddie asks the girls to order 54 new wheels for 21 skateboards and bicycles, then
x+y=21.
One scateboard has 4 wheels, then x scateboards have 4x wheels, one bicycle has 2 wheels, then y bicycles have 2y wheels. In total there should be ordered 54 wheels, so
4x+2y=54.
Solve the system of equations:
From the first equation Substitute it into the second equation:
Then
Answer: 6 scateboards and 15 bicycles.