some solution added

21. Viral Advertising/Viral Advertising.cpp

@@ -0,0 +1,16 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+int main() {
+    int n;
+    cin >> n; //Taking input
+    int resultwillbe = 2,likeaarecurrently = 2;
+    //using for loop
+    for(int i = 2; i <= n; i++){
+        likeaarecurrently*=3; //Incrementing
+        likeaarecurrently = likeaarecurrently/2;
+        resultwillbe+=likeaarecurrently;
+    }
+    cout << resultwillbe << endl; //Printing Output
+    return 0;
+}

21. Viral Advertising/Viral Advertising.py

@@ -0,0 +1,15 @@
+import math
+input_str = input()
+try:
+    n = int(input_str)
+except:
+    print("An error occurred while parsing the required input as integer.")
+if n <= 0 or n >= 51:
+    print("Bad input specified! Please try again")
+result = 0
+people = 5
+for day in range(1, n + 1):
+    half = math.floor(people / 2)
+    result += half
+    people = 3 * half
+print(result)