description: Monocarp has got an array a consisting of n integers. Let's denote k as the mathematic mean of these elements (note that it's possible that k is not an integer). 
  
  The mathematic mean of an array of n elements is the sum of elements divided by the number of these elements (i. e. sum divided by n).
  
  Monocarp wants to delete exactly two elements from a so that the mathematic mean of the remaining (n - 2) elements is still equal to k.
  
  Your task is to calculate the number of pairs of positions [i, j] (i < j) such that if the elements on these positions are deleted, the mathematic mean of (n - 2) remaining elements is equal to k (that is, it is equal to the mathematic mean of n elements of the original array a).
  
  Input
  
  The first line contains a single integer t (1 ≤ t ≤ 10^4) — the number of testcases.
  
  The first line of each testcase contains one integer n (3 ≤ n ≤ 2 ⋅ 10^5) — the number of elements in the array.
  
  The second line contains a sequence of integers a_1, a_2, ..., a_n (0 ≤ a_i ≤ 10^{9}), where a_i is the i-th element of the array.
  
  The sum of n over all testcases doesn't exceed 2 ⋅ 10^5.
  
  Output
  
  Print one integer — the number of pairs of positions [i, j] (i < j) such that if the elements on these positions are deleted, the mathematic mean of (n - 2) remaining elements is equal to k (that is, it is equal to the mathematic mean of n elements of the original array a).
  
  Example
  
  Input
  
  
  4
  4
  8 8 8 8
  3
  50 20 10
  5
  1 4 7 3 5
  7
  1 2 3 4 5 6 7
  
  
  Output
  
  
  6
  0
  2
  3
  
  Note
  
  In the first example, any pair of elements can be removed since all of them are equal.
  
  In the second example, there is no way to delete two elements so the mathematic mean doesn't change.
  
  In the third example, it is possible to delete the elements on positions 1 and 3, or the elements on positions 4 and 5.
solution: import java.io.*;
  import java.util.*;
  
  public class Mainnn {
  
      // static final File ip = new File("input.txt");
      // static final File op = new File("output.txt");
      // static {
      // try {
      // System.setOut(new PrintStream(op));
      // System.setIn(new FileInputStream(ip));
      // } catch (Exception e) {
      // }
  
      // // in = new InputReader(System.in);
      // }
  
      static long getPairsCount(long[] arr, int n, double sum) {
          HashMap<Double, Integer> hm = new HashMap<>();
          for (int i = 0; i < n; i++) {
              if (!hm.containsKey((double) arr[i]))
                  hm.put((double) arr[i], 0);
  
              hm.put((double) arr[i], hm.get((double) arr[i]) + 1);
          }
          long twice_count = 0;
          for (int i = 0; i < n; i++) {
              if (hm.get(sum - arr[i]) != null)
                  twice_count += hm.get(sum - arr[i]);
              if (sum - arr[i] == arr[i])
                  twice_count--;
          }
          return twice_count / 2;
      }
  
      public static void main(String[] args) {
          MyScanner sc = new MyScanner();
          out = new PrintWriter(new BufferedOutputStream(System.out));
  
          // out.println(result); // print via PrintWriter
          /////////////////////////////////////////////
  
          int test = sc.nextInt();
          while (test-- != 0) {
              int n = sc.nextInt();
              long[] a = new long[n];
              long sum = 0;
              for (int i = 0; i < n; i++) {
                  a[i] = sc.nextLong();
                  sum += a[i];
              }
              double sumR = (n - 2) * (sum / (double) n);
              double find = sum - sumR;
              long cnt = getPairsCount(a, n, find);
              System.out.println(cnt);
          }
      }
  
      static long power(long x, long y, long p) {
          long res = 1;
          x = x % p;
          if (x == 0)
              return 0;
          while (y > 0) {
              if ((y & 1) != 0)
                  res = (res * x) % p;
              y = y >> 1;
              x = (x * x) % p;
          }
          return res;
      }
  
      static void sortI(int[] arr) {
          int n = arr.length;
          Random rnd = new Random();
          for (int i = 0; i < n; ++i) {
              int tmp = arr[i];
              int randomPos = i + rnd.nextInt(n - i);
              arr[i] = arr[randomPos];
              arr[randomPos] = tmp;
          }
          Arrays.sort(arr);
      }
  
      static void sortL(long[] arr) {
          int n = arr.length;
          Random rnd = new Random();
          for (int i = 0; i < n; ++i) {
              long tmp = arr[i];
              int randomPos = i + rnd.nextInt(n - i);
              arr[i] = arr[randomPos];
              arr[randomPos] = tmp;
          }
          Arrays.sort(arr);
      }
  
      private static long gcd(long a, long b) {
          return (a == 0 ? b : gcd(b % a, a));
      }
  
      static class pair {
          int a;
          long b;
  
          pair(int x, long y) {
              this.a = x;
              this.b = y;
          }
      }
  
      static class comparator implements Comparator<pair> {
  
          public int compare(pair p1, pair p2) {
              if (p1.a > p2.a)
                  return 1;
              else if (p1.a < p2.a)
                  return -1;
              return 0;
          }
      }
  
      // -----------PrintWriter for faster output---------------------------------
      public static PrintWriter out;
  
      // -----------MyScanner class for faster input----------
      public static class MyScanner {
          BufferedReader br;
          StringTokenizer st;
  
          public MyScanner() {
              br = new BufferedReader(new InputStreamReader(System.in));
          }
  
          String next() {
              while (st == null || !st.hasMoreElements()) {
                  try {
                      st = new StringTokenizer(br.readLine());
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
              }
              return st.nextToken();
          }
  
          int nextInt() {
              return Integer.parseInt(next());
          }
  
          long nextLong() {
              return Long.parseLong(next());
          }
  
          double nextDouble() {
              return Double.parseDouble(next());
          }
  
          String nextLine() {
              String str = "";
              try {
                  str = br.readLine();
              } catch (IOException e) {
                  e.printStackTrace();
              }
              return str;
          }
  
      }
      // --------------------------------------------------------
  }