将数据看作字符

#include <stdio.h>

int main() {
    char a, b, c, d, e;
    scanf("%c%c%c%c%c", &a, &b, &c, &d, &e);
    printf("%c%c%c%c%c", e, d, c, b, a);
    return 0;
}

转为整数，利用除和模

#include <stdio.h>

int main() {
    float x;
    scanf("%f", &x);
    int n = x * 10;  // int x = 123.4 = 123;
    int a = n / 1000;
    int b = n / 100 % 10;
    int c = n / 10 % 10;
    int d = n % 10;
    printf("%d.%d%d%d", d, c, b, a);
    return 0;
}

#include <stdio.h>
int main() {
    float m;
    scanf("%f", &m);
    int n = m * 10;
    int a = n / 1000, b = n / 100 % 10, c = n / 10 % 10, d = n % 10;
    float sum = (d * 10 + c * 1.0 + b * 0.1 + 0.01 * a) / 10;
    printf("%.3f", sum);
    return 0;
}