int arr[5] = { 15, 30, 11, 20, 5 };

float local_sum = 0;
int i;
for (i = 0; i < sizeof(arr) / sizeof(arr[0]); i++) {
  local_sum += arr[i];
}

// Reduce all of the local sums into the global sum
float global_sum;
MPI_Reduce(&local_sum, &global_sum, 1, MPI_FLOAT, MPI_SUM, 0, MPI_COMM_WORLD);