double return type in c

1#include<stdio.h>
2double cal_per(int, int, int);
3int main()
4{
5  int a = 55, b = 66, c = 77;
6  double res;  
7  res = cal_per(a,b,c);
8  printf("%f\n",res);
9  return 0;
10}
11double cal_per(int a, int b, int c)
12{
13  double res;
14  res = (a+b+c)/300.0;
15  res *= 100;
16  return res;
17}