An automobile factory is assembling cars and they need to have all of the parts in stock at the assembly line. Each car needs four tires and a spare tire for the trunk. If they produce c cars per day, what formula shows how to calculate the number of tires, t, needed?

t = 5c

Each car needs 5 tires (4 on it and 1 spare, which add up to five). So for every one car they make they need 5 tires. Two cars would need 10 tires, 3 would need 15 tires, etc, so the number of tires can be found by multiplying the number of cars by 5. Putting this into equation form, you'd get t = 5c