There are several postage stamps of 1 yuan, 2 yuan and 5 yuan respectively. If the postage of each kind of letter is equal to the total price of 4 stamps, how many kinds of letters with different postages are there?
现有1元、2元和5元的邮票各若干枚，如果每种信函的邮资等于4枚邮票的总价，一共有多少种不同邮资的信函？
<gadget id="calculator">3 * 3 * 3 * 3</gadget>
<output>81</output>

<gadget id="calculator">81 - 1</gadget>
<output>80</output>

<result>80</result>