The <em><u>correct answer</u></em> is:
15 bicycles and 6 skateboards.
Explanation:
Let x represent the number of bicycles and y represent the number of skateboards. The first equation in our system is then
x+y = 21, since there are 21 total items in the shop.
Each bicycle has 2 tires and each skateboard has 4; this gives us the equation
2x+4y=54 (because they are ordering 54 tires).
This makes our system
We will use elimination to solve this; we first need one of the variables to have the same coefficient in both equations. We will make x the same, and we will do this by multiplying the first equation by 2:
Now we will subtract the second equation from the first one:
Divide both sides by -2:
-2y/-2 = -12/-2
y = 6
There were 6 skateboards. Substituting this into the first equation,
x+6 = 21
Subtract 6 from each side:
x+6-6 = 21-6
x = 15
There were 15 bicycles.