c++问题，就两题，要考试，大神帮帮忙

亲，题目？

#include
#include

int main(){

// part1:
{
int a[5][5]={1,2,3,4,5,6,7,8,9,10,
11,12,13,14,15,16,17,18,19,20,
21,22,23,24,25};
int b[5][3]={3,0,16,17,-6,9,0,23,-4,9,7,0,4,13,11};
int c[5][3];
int i,j,k;
for (j=0;j<5;j++){
for (i=0;i<3;i++){
c[j][i]=0;
for (k=0;k<5;k++) {c[j][i]=c[j][i] + a[j][k]*b[k][i];}
}
}

for (j=0;j<5;j++){
for (i=0;i<3;i++){
printf("%d ",c[j][i]);
}
printf("\n");
}

} // end part1

//part2:
{
int score[100];
int i,n_student=50;
int n1=0;
double sum=0,ave;
int nn[10];
srand(12345);
for (i=0;iprintf("score < 60 student ID:\n ");
for (i=0;iprintf("%d ",i);n1=n1+1;
}
printf("\nNumber of students who score < 60 is: %d\n",n1);

for (i=0;iave = sum / n_student;
for (i=0;iave) {
printf("id=%d ",i);n1=n1+1;
}
printf("\nNumber of students who score > ave is: %d\n",n1);

for (i=0;i<10;i++) nn[i]=0;
for (i=0;iif (score[i]==100)nn[9]=nn[9]+1; else nn[(score[i]-1)/10]++;
}

printf("0 - 10: %d\n",nn[0]);
for (i=1;i<10;i++) {
printf("%d - %d: %d\n",i*10+1,i*10+10,nn[i]);
};
} //end part2
return 0;
}
// 匆匆写了一下。成绩分档部分你要检查一遍。

你能解释下第一题吗 2个矩阵相乘是怎么乘的呢？
第二题会做 第一题题意不清楚
还有第二题的各分数段是哪些分数段呢？有说名吗？
我先跟你敲第二题