生成函数系数计算问询：AMC2023 10B第11题生成函数中$x^{80}$的系数求解

Suzanne went to the bank and withdrew dollar $800$. The teller gave her this amount using dollar $20$ bills, dollar $50$ bills, and dollar $100$ bills, with at least one of each denomination. How many different collections of bills could Suzanne have received? (AMC 2023 10B/11)

我看到这道题的解法5用生成函数得到了这个表达式：
$$(x^{82}-1)(x{85}-1)(x^{90}-1)(x2-1)^{-1}(x5-1)^{-1}(x{10}-1)^{-1}.$$

不过我搞不懂怎么求这个表达式里$x^{80}$的系数。我知道前三个项可以忽略，但之后就不知道该怎么做了——总不能又绕回其他解法里用的丢番图方程/星与条的方法吧？

备注：内容来源于stack exchange，提问作者Starlight