The answer is the same symbol for any 2 digit number. Look at the chart and try any 2 digit number. The answer is always a multiple of 9, up to 81. All the symbols for 9, 18 .. 81 in the chart are the same.
When you click on try again, it scrambles the symbols around, so the answer symbol changes. So the illusion is that it's a different answer when you try again with a different number.
Somebody posted a different link to this before. It's still pretty cool until you figure it out.
Let x = an integer from 1 .. 9
Let y = an integer from 0 .. 9
10x + y is the number you pick. (for example, 23, x = 2 and y = 3)
10x + y - (x + y) is the subtraction you do.
10x + y - x - y is the same as the above line
10x - x + y - y is the same as the above line
y - y is zero. 10x - x is 9x.
9x is divisible by 9. (For 23, 9x is 9 x 2 or 18).
-David
