IVE WASTED 60 POINTS I will mark as brainlest!! Plan an investigation to study the effects of exercise on heart rate. Identify the
independent, dependent and controlled variables. Plan the experimental
group(s) and control. Determine the sample size and number of trials as well.

The correct answer 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.