Encuentre el entero que falta en una array si se da la media

Dada una array de tamaño N-1 y la media de N elementos (no se proporciona un elemento). Necesitamos encontrar el valor faltante X en la array.
Ejemplos: 
 

Input : a[] = {2, 4, 20}
       Mean = 9
Output : Missing Element = 10 
Explanation : Mean of (2, 4, 20, 10) is
(2 + 4 + 20 + 10)/4 = 9

Sea x el elemento faltante
Media = (a ₁ + a ₂ + a ₃ .. + X + ..a _n )/N. 
Entonces (a ₁ + a ₂ + a ₃ .. + X + ..a _N ) = Media*N . 
Elemento faltante x = (Media*N – (a ₁ + a ₂ + a ₃ ….a _N ))
 

// C++ program to find missing element in a
// given array from mean.
#include <bits/stdc++.h>
using namespace std;
 
// Size of a[] is N - 1 (one element missing)
int findMissing(int a[], int N, int mean)
{
  // Find sum of array elements
  int sum = 0;
  for (int i = 0; i < N - 1; i++)
    sum += a[i]; 
 
  return (mean * N) - sum;
}
 
int main() {
  int a[] = {25, 65, 80};
  int mean = 50;
  int n = sizeof(a)/sizeof(a[0]);
  cout << "The missing element : "
       << findMissing(a, n+1, mean)  << endl;
  return 0;
}

// Java program to find missing element
// in a given array from mean.
import java.io.*;
 
class GFG
{
    // Size of a[] is N - 1 (one element missing)
    public static int findMissing(int a[], int N, int mean)
    {
        // Find sum of array elements
        int sum = 0;
        for (int i = 0; i < N - 1; i++)
        sum += a[i];
 
        return (mean * N) - sum;
    }
     
    // Driver code
    public static void main (String[] args)
    {
        int a[] = {25, 65, 80};
        int mean = 50;
        int n = a.length;
        System.out.println("The missing element : "
                           + findMissing(a, n + 1, mean));
         
    }
}
 
// This code is contributed by upendra bartwal

# Python3 code to find missing element
# in a given array from mean.
 
# Size of a[] is N - 1
# (one element missing)
def findMissing( a , N , mean ):
 
    # Find sum of array elements
    sum = 0
    for i in range(N - 1):
        sum += a[i]
     
    return (mean * N) - sum
 
# Driver Code
a = [25, 65, 80]
mean = 50
n = len(a)
print("The missing element : ", end = '')
print(findMissing(a, n+1, mean))
 
# This code is contributed by Sharad Bhardwaj.

// C# program to find missing element
// in a given array from mean.
using System;
 
class GFG {
     
    // Size of a[] is N - 1
    // (one element missing)
    public static int findMissing(int[] a, int N, int mean)
    {
         
        // Find sum of array elements
        int sum = 0;
        for (int i = 0; i < N - 1; i++)
            sum += a[i];
 
        return (mean * N) - sum;
    }
 
    // Driver code
    public static void Main()
    {
        int[] a = { 25, 65, 80 };
        int mean = 50;
        int n = a.Length;
        Console.WriteLine("The missing element : "
                    + findMissing(a, n + 1, mean));
    }
}
 
// This code is contributed by vt_m.

<?php
// PHP program to find missing
// element in a given array
// from mean.
 
// Size of a[] is N - 1
// (one element missing)
function findMissing($a, $N, $mean)
{
     
    // Find sum of array elements
    $sum = 0;
    for ($i = 0; $i < $N - 1; $i++)
        $sum += $a[$i];
     
    return ($mean * $N) - $sum;
}
 
// Driver Code
$a = array(25, 65, 80);
$mean = 50;
$n = count($a);
echo "The missing element : "
    .findMissing($a, $n + 1, $mean);
         
// This code is contributed by Sam007
?>

<script>
 
// Javascript program to find
// missing element in a
// given array from mean.
 
 
// Size of a[] is N - 1
// (one element missing)
function findMissing( a, N, mean)
{
// Find sum of array elements
let sum = 0;
for (let i = 0; i < N - 1; i++)
    sum += a[i];
 
return (mean * N) - sum;
}
 
    // Driver Code
     
    let a = [25, 65, 80];
    let mean = 50;
    let n = a.length;
    document.write("The missing element : "
        + findMissing(a, n+1, mean) + "</br>");
     
     
</script>

Producción: 

The missing element : 30