You can define sum 1 as a float, or you can force type conversion by setting aver1= (float) sum1/3.

int a[3]={80，60，90 }；

Floating point aver1;

float sum 1 = a[0]+a[ 1]+a[2]；

aver 1 = sum 1/3；

Printf ("The average score of student 1 is %f", Aver1);

printf(" \ n ")；

or

int a[3]={80，60，90 }；

Floating point aver1;

int sum 1 = a[0]+a[ 1]+a[2]；

aver 1 =(float)sum 1/3；

Printf ("The average score of student 1 is %f", Aver1);

printf(" \ n ")；