第三题该怎么解决啊，拜托了

代码如下：

#include <iostream>
#include <string>
using namespace std;

class StudentScore
{
private:
    int id;
    string name;
    int sc; //课程成绩
public:
    StudentScore(){};
    void setScore(int s){sc = s;}
    int getScore(){return sc;}
    void setId(int ids){id = ids;}
    int getId(){return id;}
    void setName(string nn){name = nn;}
    string getName(){return name;}
    static void Caculate(StudentScore a[],int n,int &total, float &avg);
};

void StudentScore::Caculate(StudentScore a[],int n,int &total, float &avg)
{
    int i;
    total = 0;
    avg = 0.0;
    for (i=0;i<n;i++)
    {
        total += a[i].getScore();
        avg += a[i].getScore();
    }
    avg /= n;
}

int main()
{
    StudentScore a[5];
    int total = 0;
    float avg = 0;
    a[0].setScore(33);
    a[1].setScore(44);
    a[2].setScore(55);
    a[3].setScore(66);
    a[4].setScore(77);

    StudentScore::Caculate(a,5,total,avg);
    cout << "total=" << total << ",avg=" << avg << endl;
    return 0;
}