This is a fantastic trick. I'm usually not a fan of math tricks but this one is very different. It
totally doesn't feel like a math trick and the fact that they write the prediction is so disarming.
It is hypothetically impromptu and you can do this with everything borrowed although if you really
want to do this you should probably carry around your own change just in case. Most people don't
have enough change on their own and I haven't found spectators being too eager to pool their cash
together.
There is a slight risk that your first spectator make take a completely wrong
number of coins and then the way to take care of that can hypothetically not go nearly as planned
and end up seeming very forced, but if you have a decent amount of experience performing you
shouldn't need to worry about it. I would not recommend this trick to anyone who doesn't have
significant practice performing in front of real audiences otherwise their directions won't seem
inconsequential enough. (I'm sorry if that doesn't make so much sense. I'm trying to avoid anything
that can be a tip-off to the method.)
This method is a lot of fun and there are a lot of
layers to this. It's really hard to deconstruct. You'll have a lot of fun doing this.