Answer:
$3 per notebook.
Explanation:
i would start by taking the two notebooks away from the imaginary cart.
then a $30 backpack=10 notebooks.
if so, then, each notebook=$3 because 3x10=30.
if you re-add the two other notebooks then it would be twelve $3 notebooks=$36 ($30 backpack and two $3 notebooks) and 12x3=36 and 36=36