Final answer:
To find the total cost of trash bags for 120 days, divide the cost per day by the number of days and multiply by the number of days.
Step-by-step explanation:
To find the total cost of the trash bags for 120 days, we first need to determine the cost of a trash bag for one day. Since eight trash bags last for thirty days, we can calculate the cost per day by dividing the total cost by the number of days. $8.40 / 30 = $0.28.
Next, we need to calculate the number of trash bags needed for 120 days. If one trash bag lasts for one day, then for 120 days we will need 120 trash bags.
Finally, we can find the total cost by multiplying the cost per day by the number of days. $0.28 * 120 = $33.60.
Therefore, the student will pay $33.60 for a supply of trash bags for 120 days.