C :

#include <stdio.h>
int main()
{
    double m, money;
    scanf("%lf", &m);
    if(m > 0 && m <= 3)
    {
        money = 10;
        printf("%.2lf\n", money);
    }
    else if(m > 3 && m <= 5)
    {
        money = 10 + (m - 3) * 1.8;
        printf("%.2lf\n", money);
    }
    else if(m > 5)
    {
        money = 10 + (5 - 3) * 1.8 + (m - 5) * 2.7;
        printf("%.2lf\n", money);
    }
    return 0;
}

C++ :

#include<cstdio>
using namespace std;
int main()
{
  double n,s,n1,n2;
  scanf("%lf",&n);
  if (n<=3) 
  {
     s=10;
  }
  else 
  {
    if (n<=5)
    {
      s=10+(n-3)*1.8;
    }
    else 
    {
        s=10+2*1.8+(n-5)*2.7;
    } 
  }
  printf("%.2lf",s);
  return 0;
}