Margo brought 7 pens from a book store some pens cost $3.00 EACH and the rest cost $4.00 . If she paid a total of 23.00 for the pens how many of the $3.00 pens did she buy

5

Step-by-step explanation:

Margo bought 7 pens for $23.

Some pens are $3 and some $4.

We need to find how many $3 pens she bought.

Let the number of $3 pens be represented by p and let the number of $3 pens be represented by r. So:

p = $3  and r = $4

Now we know 7 pens were purchased so is we add the number of $3 pens purchased to the number of $4 pens purchased, we will get 7.

p + r = 7

We know that the total cost of the pens is $23 so that equation would be found by takign the $3 pen multiplied by the number of $3 pens purchsed added to the $4 pen multiplied by the number of $4 pens purchsed, which equals $23.

$3p + $4r = $23

Now we have 2 equations and 2 unknowns.

p + r = 7

$3p + $4r = $23

Let's solve for r in the first equation.

p + r = 7     Subtract p from both sides.

p - p + r = 7 - p   The p on the left cancels.

r = 7 - p

Now that we know r, we can substitute it into the second equation and solve for p.

$3p + $4r = $23

3p + 4( 7 - p) = 23       Multiply it out.

3p + 4*7 - 4*p = $23

3p + 28 - 4p = 23      Combine like terms.

3p  - 4p + 28 = 23

- 1p + 28 = 23       Subtract 28 from both sides.

- p + 28 - 28 = 23 - 28  

- p = - 5   Divide each side by -1

- p/- 1 = - 5/ - 1       The negative cancels on each side.

p = 5

We can see Margo bought 5 $3 pens.

For fun, let's solve for how many $4 pens she bought. We know p, so we plug it in the first equation and solve for r.

p + r = 7

5 + r = 7  Subtract 5 from each side

5 - 5 + r = 7 - 5

r = 7 - 5

r = 2

So Margo bought 5 $3 pens and 2 $4 pens!

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