Nice Parlor Trick - Spoiler in here...
The flash is made well - nice colors, moves smoothly, and lets you change the color of the ball - all of which is pleasant, but nothing more than window dressing.
How does it know the symbol? That's the simplicity of it. Any 2-digit number, when you take away the sum of the digits, leaves a value that is a multiple of 9. If you look closely at all the multiples of 9 (9,18,27, etc) they all have the same symbol of "Taurus" The Bull. So, mathematically, you always are steered into a known value - and a predetermined symbol.
I gave it a 7/10 because it's well done, but an "oldie" - and a 4/5 'cause it should pass - and make most first-time viewers think "hey, how'd it know that?"