Define a function called sum, for a given an array and an int representing its size, if the size is at least 3, then sum up all elements whose indices are a multiple of 3 and return that value, otherwise, return 0.

Written in C++

int sum(int arr[], int size) {

 int total = 0;      

 if(size>=3)  {

  for (int i = 0; i < size; ++i) {

     if(i%3 == 0)      {

     total += arr[i];    

     }

  }

 }

  return total;

}

Explanation:

This line defines the function

int sum(int arr[], int size) {

This line initializes sum to 0

 int total = 0;      

This line checks if size is at least 3

 if(size>=3)  {

This loop iterates through the array

  for (int i = 0; i < size; ++i) {

The following adds multiples of 3

     if(i%3 == 0)      {

     total += arr[i];    

     }

  }

 }

This returns the calculated sum

  return total;

}

See attachment for full program