#include<bits/stdc++.h>
using namespace std;
signed main(){
	double a;
	cin>>a;
	if(a<=6)a*=2;
	else if(a<=10)a=12+(a-6)*4;
	else a=28+(a-10)*8;
    printf("%.1lf",a);
}

C++ :

#include<bits/stdc++.h>
using namespace std;
double n,m;
int main()
{
cin>>n;
m=n*2;
if(n>6)
	m+=(n-6)*2;
if(n>10)
	m+=(n-10)*4;
int a,b;
a=m;
b=m*10;
b=b%10;
cout<<a;
cout<<'.';
cout<<b;

return 0;
}

Python :

# coding=utf-8
s=float(input())
if s<=6:
    print(s*2)
elif s<=10:
    print(6*2+(s-6)*4)
else:
    print((s-10)*8+12+16)