Maria bought 4 notebooks and some pens at the school bookstore for a total of $13. Each notebook costs $2 and each pen costs $0.50. How many pens did Maria buy?
For word problems about buying multiple items, first translate the words into simple arithmetic: multiply the quantity of each known item by its unit price to get its total cost, subtract these known totals from the overall amount to find what is left for the unknown item, then divide that remaining amount by the unknown item’s unit price. Be careful to distinguish between the number of items and their total cost, and handle decimals like $0.50 by thinking of them as fractions (e.g., $0.50 = 1/2) if division by decimals feels tricky.
Hints
Separate notebooks and pens
First focus on the notebooks: how much money did Maria spend on the 4 notebooks if each one costs $2?
Find the money left for pens
After you find the total cost of the notebooks, subtract that amount from $13 to find how much money was used for pens.
Use the cost per pen
You know how much money was spent on pens and that each pen costs $0.50. What operation will tell you how many pens that amount of money can buy?
Desmos Guide
Compute the number of pens directly
In a Desmos expression line, type (13 - 4*2) / 0.5 and press Enter. The value that Desmos outputs is the number of pens Maria bought.
Step-by-step Explanation
Find the total cost of the notebooks
Each notebook costs $2 and Maria bought 4 notebooks.
So the total cost of the notebooks is:
Maria spent $8 on notebooks.
Find how much money was spent on pens
Maria spent a total of $13.
You already found she spent $8 on notebooks.
Subtract to find how much was spent on pens:
So Maria spent $5 on pens.
Use the price per pen to find the number of pens
Each pen costs $0.50 (fifty cents).
To find how many pens she bought, divide the total amount spent on pens by the cost per pen:
Think of $0.50 as . Then
So Maria bought 10 pens.