20，求助！

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int a,b,c,n;
    char last,s[20],ans[20];
    scanf("%d",&n);
    getchar();
    while(n--)
    {
        fgets(s,sizeof(s),stdin);
        if(s[0]=='a'||s[0]=='b'||s[0]==c)
        {
            last=s[0];
            s[0]=' ';
        }
        sscanf(s,"%d %d",&a,&b);
        switch(last)
        {
            case 'a':c=a+b;sprintf(ans,"%d+%d=%d",a,b,c);break;
            case 'b':c=a-b;sprintf(ans,"%d-%d=%d",a,b,c);break;
            case 'c':c=a*b;sprintf(ans,"%d*%d=%d",a,b,c);break;
        }
        printf("%s\n%d\n",ans,strlen(ans));
    }
}