其实吧，挺简单的

#include <iostream>//hetao3097453
#include <iomanip>
using namespace std;
int main()
{
    double a,b,c;
    cin >> a;
    b = a * 3.0;
    c = b - 8.0;
    cout << b << endl;
    cout << c << endl;
    cout << fixed << setprecision(1) << (a + b + c) / 3 << endl;
    return 0;
}

其实吧，极其简单

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int x;
    cin >> x;
    int a=x*3;
    int b=a-8;
    cout << a << endl << b << endl;
    cout << fixed << setprecision(1) << (a+b+x)/3.0;
    return 0;
}

已AC

#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
    int x, y, z;
    cin >> x;
    y = 3 * x;
    z = y - 8;
    cout << y << endl << z << endl << fixed << setprecision(1) << (x + y + z) / 3.0;
    return 0;
}

过辣！（脑残人士的欢呼）

    double rabbit,fox,sheep;
    cin>>rabbit;
    fox=rabbit*3;
    sheep=fox-8;
    cout<<fox<<endl<<sheep<<endl<<fixed<<setprecision(1)<<1.0*(rabbit+fox+sheep)/3;

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int a , b , c;
    double v;
    cin >> a;
    b = a * 3;
    c = b - 8;
    v = (a + b + c) / 3.0;
    cout << b << endl << c << endl;
    printf("%.1f",v);
    return 0;
}

A