Cost of each notebook = $1.75
As we can see in the question, sebastian bought the package of pens for $3.5, so the quantity is only one. Also we know that he bought total of 4 notebooks. Individual price of notebook is what we are required to find. However, we know total which we paid, so assuming the cost of a notebook as a variable, we can form an equation and calculate the cost of 1 notebook.
Let c = the cost of each notebook
Then total cost = $10.5 = $3.5 + [tex]4 \times c[/tex]
[tex]4 \times c[/tex] = $10.5 - $3.5
[tex]4 \times c[/tex] = $7c = $[tex]\frac{7}{4}[/tex]c = $1.75
Hence the cost of each notebook = $1.75
