I had $21 to spend on two notebooks after buying them I had $13 how much did each notebook cost

I had $21 to spend on two notebooks after buying them I had $13 how much did each notebook cost?

If you had $13, leftover, this means you spent the following on notebooks:
$21 - $13 = $7

Cost per notebook = Total Notebook spend / Total Notebooks
Cost per notebook = $7/2
Cost per notebook = $3.50