#include <stdio.h>
int main(void)
{
    float inch;
    float cm = 2.54;
    printf("请输入 inch\n");
    scanf("%f", &inch);
    printf("%f inch = %f cm\n", inch, inch * cm);
    return 0;
}

#include <stdio.h>
int main(void)
{
     float inch , cm ;

     printf("请输入inch \n");//先知道输入多少inch
     scanf("%f",&inch);

     cm = 2.54 * inch ;//再转换成cm

     printf("%f inch = %f cm\n", inch, cm);//输出
     return 0 ;

}