Suppose you bought a pizza for $8.00. You also buy a salad, but you cannot remember how much you paid and you can't find a receipt. You know that the total amount you spent was $10.00. Which equation would be best to use to find the cost of the salad?
Because we already know the cost of the pizza ($8) and the only one we don't know the cost of is the salad, but we know the total of both items was $10.
