C#

In my interview following question was asked but I was unable to answer. Please help me in solving it.

The algorithm below is a simple checksum formula used to validate a variety of identification numbers.

Your Task: Write a class that can be used to add a check digit to any number.
The check digit is calculated as follows:

1.	Counting from the right-most digit and moving left, double every alternate digit starting with the first one. For any digits that thus become 10 or more, take the two numbers and add them together. For example, 111 becomes 212, while 876 becomes 773.

2.	Add all these digits together. For example, if 111 becomes 212, then 2+1+2 is 5; and 873 becomes 773, so 7+7+3 is 17.

3.	The check digit is: 10 – (<the sum above> % 10), where % is the modulus operator (gives the remainder after division). For example, the check digit for 111 is 10 - the remainder of (2+1+2)/10, which is 5. The check digit for 876 is 10 - (7+7+3)%10, which is 3.