Let the cost of one pen be $p and that of one notebook be $q.
At the beginning of the school year, Caitlin paid $22 to buy four pens and three notebooks.
Therefore, 4p + 3q = 22. Let’s call this equation as equation (1)
Two months later, she decided to buy six more of the same pens and five more of the same notebooks, at the same price, spending $35 on this purchase.
Hence, 6p + 5q = 35. Let’s call this equation as equation (2)
There are two unknowns and two independent equations in these unknowns. The two equations can be solved simultaneously for p and q.
Mutliplying equation (1) by 3 and equation (2) by 2, we have,
12p + 9q = 66
12p + 10q = 70
Subtracting the equations, we have, q = 4. This is the cost of one notebook, which is what we are trying to find out.
There is no need to calculate the value of p.
The correct answer option is D.