Teaching Math to Kids at the Grocery Store - Verywell Family If you've never done this, divide the cost by the quantity to determine the unit price. For example, you can buy 2 pounds of grapes for $2.00 or 3 pounds of grapes for $3.50. $2.00/2= $1.00 per pound vs. $3.50/3=$1.16 per pound. Thus, the cost of 2 pounds is the better price. Compare the actual cost of your groceries to your child's earlier ...