24. To mail a letter in the U.S.A. it costs 20 cents for the first ounce and 18 cents for each additional ounce or fraction thereof. Find a formula involving the greatest integer function for the cost of mailing a letter. Could it possibly cost $\$ 1.08$ or $\$ 1.28$ to mail a letter?
24. $20+18[\mathrm{x}-1], \$ 1.08$ no, $\$ 1.28$ yes