How many bicycles and how many skateboards are in the shop? Show your work.

15 bicycles, 6 skateboards

Step-by-step 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

          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.

15 bicycles, 6 skateboards

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