The way people are going on you'd think calculus or some higher form of maths were involved, rather than simple subtraction and division. You can all subtract and divide I take it.
Well, some of the calculation will require a bit of algebra and some understanding of the way to calculate probabilities and combinations. But, you're right that most of the stuff you deal with on the table is grade school math.
I've actually had 4th graders handling it pretty easily. They didn't know it was poker (we called it low cards) and there was no betting or chips. It was stuff where I would deal myself a hand like 10-9-6-3-2 and deal them a K-8-7-6-4 and they would have to write a fraction for how many cards would give them a lower set when they threw away the King (not hand... because, you know, we're not playing poker... lol) and then guess how many cards they would have to throw away and redraw (unlimited) until they won. They idea was the try and pick the lowest number that still had you get a winning card.
The trick, as I told them, to the last one was to divide the total unknown cards by the number of cards that helped them (because division was another skill they work on at that age). They were initially amazed at how often it was within that number of cards, but eventually they just took it for granted.
That's pretty complex number sense, for a 9 year old, but with a little practice, they got really good at it. If a 9 year old could get it, so can most people here.