Answer:
15 bicycles, 6 skateboards
Explanation:
We are looking for the number of skateboards and the number of bicycles. Those two numbers are our unknowns.
We define variables for those two numbers.
Let s = number of skateboards.
Let b = number of bicycles.
A skateboard has 4 wheels. s number of skateboards have 4s wheels.
A bicycle has 2 wheels. b bicycles have 2b wheels.
The total number of wheels is 4s + 2b.
The total number of wheels is 54, so our first equation is
4s + 2b = 54
The total number of skateboards and bicycles combined is s + b.
We are told the total number of skateboards and bicycles combined is 21.
The second equation is
s + b = 21
We have a system of two equations in two unknowns.
4s + 2b = 54
s + b = 21
We can solve it by the elimination method.
Rewrite the first equation below.
Multiply both sides of the second equation by 2 and write it below that. Then add the equations.
4s + 2b = 54
(+) -2s - 2b = -42
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2s = 12
s = 6
There are 6 skateboards.
Now we substitute 6 for s in the second original equation and solve for b.
s + b = 21
6 + b = 21
b = 15
There are 15 bicycles.
Answer: 15 bicycles, 6 skateboards