邮票凑数

由于 $\gcd(5,21)=1$，不能表示的最大数为 Frobenius 数：

$$5\cdot 21-5-21=79.$$

因此最大不能凑出的金额是 79。

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Original Explanation

The values $21$, $42$, $63$, and $84$ are the smallest numbers equivalent to $1$, $2$, $3$, and $4$ modulo $5$, respectively. By adding multiples of $5$ to these values, we can create any number equivalent to those in modulo $5$. The largest value that cannot be formed is the highest number that is $4$ modulo $5$, which is $79$. Thus, $79$ is the largest integer we cannot create.