where ${X}_{1},{X}_{2},\dots $ are independent, identically distributed continuous random variables with mean $0$ and variance ${\sigma}^{2}$. (Note that this is an additive assumption about the change in a stock price. In the binomial tree models, we assumed that a stock’s price changes by a multiplicative factor up or down. We will have more to say about these two distinct models later.) Suppose that a stock’s price today is $100$. If ${\sigma}^{2}=1$, what can you say about the probability that after $10$ days, the stock’s price will be between $95$ and $105$ on the tenth day?