How do you solve the game where you Pick any two digit number then subtract the first digit then subtract the second digit and guess the answer?

Given a two digit number --> xy

The numerical value of that number is 10x + y (The x is in the ten's place and the y is in the one's place.)

So, if you subtract each digit from the original number, here is the resut:

10x + y - x - y = 9 x

Based on that simple algebra, the answer will always be nine times the first digit. People often tend to amaze others by making a seemingly random chart of numbers and prizes. THey claim to guess the prize that the person gets based on his number. The real trick is that All multiples of 9 are matched up with the same prize!