import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.Scanner;

// a/b转为小数，精确到小数点后c位
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int count = 1;
        while (sc.hasNext()) {
            String[] strings = sc.nextLine().split(" ");
            int a = Integer.parseInt(strings[0]);
            int b = Integer.parseInt(strings[1]);
            int c = Integer.parseInt(strings[2]);
            if (a == 0 && b == 0 && c == 0) {
                return;
            }
            double res = 1.0 * a / b;
            BigDecimal num = new BigDecimal(Double.toString(res));
            // 需要截尾处理，而不是四舍五入
            num = num.setScale(c, RoundingMode.FLOOR);
            System.out.println("Case " + count + ": " + num.toString());
            count++;
        }
    }
}

你后来不是过了吗(

我想你已经知道了答案

res 变量精度损失了，res 就该直接上高精度