number_average.cpp

static void main(char*[] args) {
  
  int[] numbers = {11,12,13, 21,22,23, 31,32,33};
  
  int intervalMin = 10;
  int intervalMax = 20;
  
 cout << "10-20: Average is " << getAverage(numbers, 10, 20) << endl;
 
}

static float getAverage(int[] numbers, int intervalMin, int intervalMin) {
  long sumOfNumbersInInterval = 0;
  long countOfNumbersInInterval = 0;
  
  for (int i = 0; i < numbers.length; i++) {
    if (numbers[i] > intervalMin && numbers[i] < intervalMax) {
      sumOfNumbersInInterval += numbers[i];
      countOfNumbersInInterval += 1;
    }
  }
  return sumOfNumbersInInterval / countOfNumbersInInterval; 
}