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The equations area:

x+y=12; x=number of notebooks and y= number of pens

2.5x + y = 24

Y=24 - 2.5x

x + 24 - 2.5x =12

12 = 1.5x

x=8 notebooks

y = 4 pens

Anthony, we are going to have to come up with two equations and do some substitution.

Let's say that n= the number of notebooks and p= the number of pens.

We are given that a total of 12 items were bought, so n+p=12.

Using the prices that we have, our second equation is 2.50n+1.00p=24.00 remembering that we can take off zeroes to make our work easier and that 1 times anything is itself, we can write the equation as

2.5n+p=24

Now, going back to the first equation, since p is alone in the second equation, let's solve for p in the first. We get that p=12-n.

Subsitute this "value" for p in the second equation. We end up with

2.5n+(12-n)=24, or 2.5n+12-n=24

Combining like terms, we get 1.5n+12=24. 1.5n=12, so n=8.

She bought 8 notebooks.

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