cal.c

#include<math.h>
#include<stdio.h>

void main(){
  int i,times;
  float vo,vi;
  printf("please input Vi\n");
  scanf("%f", &vi);
  times = 1;
  for(i = 0; i < 4; i++) {
    times = times*10;
    printf("3*%d:",times);
    scanf("%f", &vo);
    printf("result:%lf\n\n", 20*log10((double)vo/vi));
    printf("5*%d:",times);
    scanf("%f", &vo);
    printf("result:%lf\n\n", 20*log10((double)vo/vi));
    printf("7*%d:",times);
    scanf("%f", &vo);
    printf("result:%lf\n\n", 20*log10((double)vo/vi));
    printf("10*%d:",times);
    scanf("%f", &vo);
    printf("result:%lf\n\n", 20*log10((double)vo/vi));
  }
}