Theory:

In the previous example, we used the letter \(n\) to denote a variable. There is no special thing about the letter \(n\). We can use any small letter alphabets to represent variable.
 
Any small letter \(a, b, c, d,..., x, y, z\) can be used to represent variables.
Suppose Aravind, Aakash and Arun are going to a shop to buy chocolates. The cost of one chocolate is \(₹6\). They are planning to buy chocolates for their family members. There are \(4\) members in Aravind's family, \(6\) members in Aakash's family and \(5\) members in Arun's family.
 
Think, is it possible to form a general rule with the help of variable and find the amount required for Aravind, Aakash and Arun to buy chocolates for their family?
 
Yes, we can form the rule.
 
Because the cost of the chocolate is \(₹6\).
 
Let the number of members in their family be \(x\).
 
Here the variable \(x\) can take any value from \(1, 2, 3,...\).  Because the number of members in a family can be any positive number.
 
The total cost of chocolates in rupees \(= 6×x = 6x\).
 
There are \(4\) members in Aravind's family.
 
Substituting \(x = 4\), the total cost becomes \(6x = 6 × 4 = 24\).
 
There are \(6\) members in Aakash's family.
 
Substituting \(x = 6\), the total cost becomes \(6x = 6 × 6 = 36\).
 
There are \(5\) members in Arun's family.
 
Substituting \(x = 5\), the total cost becomes \(6x = 6 × 5 = 30\).