21. (10 points) When Xiao Hong was organizing her change purse, she found that there were a total of 25 coins with denominations of 1 cent, 2 cents, and 5 cents, with a total value of 0.60 yuan. Then, the maximum number of 5-cent coins is $\qquad$.

将上面的文本翻译成英文，请保留源文本的换行和格式，直接输出翻译结果。 

Note: The instruction at the end is not part of the translation but is included to clarify the request. The actual translation is provided above.
21. (10 points) When Xiaohong was organizing her change purse, she found that there were a total of 25 coins with denominations of 1 cent, 2 cents, and 5 cents, with a total value of 0.60 yuan. The maximum number of 5-cent coins is $\qquad$ 8.

【Solution】Solution: Since 0.60 yuan $=60$ cents,
Let the number of 1-cent, 2-cent, and 5-cent coins be $x$, $y$, and $z$ respectively, then we have $x+y+z=25, x+2 y+5 z=60$. Subtracting the first equation from the second gives $y+4 z=35$. To maximize the number of 5-cent coins, i.e., to maximize $z$, $y$ must be minimized. Since 35 is an odd number, $y$ must be an odd number,

When $y=1$, $z$ is not an integer,
When $y=3$, $z=8$,
So $z=8$;
Answer: The maximum number of 5-cent coins is 8;
Therefore, the answer is: 8.