How to combine probabilities? If the outcome of a market could be expressed as a probability it might be: 
Outcome - Description - Probability as a %


*

*Up a lot 20% (a move of say more than 10%) 

*Down a lot 20% 

*Up a bit 20% (a move of between 0 and 10%) 

*Down a bit 20% 

*Sideways 20% 


So the probability of any single outcome is 1/5 or 20%. 
Please could some one educate me on the math of adding another and subsequent markets?
 A: OK, let me say something about the multinomial distribution since I brought it up.
Suppose I have 2 dice both with 5 faces each. Assuming that the dice are fair, the probability of one particular face turning up with one die is indeed $1/5$ or 20%.
Now, let's ask what happens when we throw two dice and ask ourselves what the probability is of having them both showing the same number. Two ones could come up with a probability of $1/25$, two twos could turn up with the same probability, etc... the total probabiliy would thus be $5 \times 1/25$ or $1/5$. A quicker way to see this, whatever comes out for the first die is setting the bar for the second die. So the second die has $1/5$ of coming out right.
Let's ask the same question for three dice. The probability will be $1/25$ by an analoguous reasoning.
The multinomial distribution allows us to compute much more. It can compute the probability of a particular number combination coming out if we don't care about the order in which the numbers come out. This probability for $n$ fair dice with $p$ sides each is
$$\mathcal{P}(n_1,\ldots,n_p)=\frac{n!}{n_1!n_2!\ldots n_p!} \frac{1}{p^n}$$
Take for instance the probability for throwing (1,1,2) with 3 5-sided dice, that is two ones and one two:
$$\mathcal{P}(2,1,0,0,0)=\frac{3!}{2!1!0!0!0!} \frac{1}{5^3} = \frac{3}{125}$$

What do you mean by market here? You mean something like the Dow Jones Industrial average?
If that's what you mean, then the way a probability would be expressed would be something like this. Denoting the Dow Jones Industrial Average in function of time as $D(t)$, we can ask what is the probability to have a rise of 10% over a certain time interval $\Delta t$. Say this probability is 5%. This would be expressed as:
$$ \mathbb{P}[D(t+\Delta t)-D(t) > 0.1 D(t)] = 0.05$$
Now, this is just notation. No actual information has been put in. If you're asking how can we model the Dow Jones Industrial Average, I'm afraid this is not something that can be given a quick answer to. I'd suggest you start reading up on the subject because it involves a lot of math. Maybe an easy start:
http://en.wikipedia.org/wiki/Stock_market
Expect the learning curve to be steep though.
The important thing to understand in your case though is that just because you can express something with % it doesn't mean it is a probability. It could correspond to a relative change, for instance, a relative change in the value of a stock. That is not a probability. But there is a probability associated to the occurance of that change.
