# What is the probability of touching point A first?

The probability of a stock touching a point A which is below the current spot price is 35%, and the probability of the stock touching a point B which is above the current spot price is 20%.

How can I calculate the probability of the stock touching point A(but B must not have been touched before A is touched)?

• This is a bit difficult to answer as you haven't specified a model for your stock! Sep 20, 2021 at 6:45
• If you don’t mind, please what do you mean by model? Is it the distribution?
– user59094
Sep 20, 2021 at 15:21
• Yes, I mean the "distribution". Is your stock assumed to be log normal (e.g. Black-Scholes)? Or normal (Bachelier)? Or other distributions? Sep 21, 2021 at 6:28
• Ok assuming a normal distribution, can there be a solution?
– user59094
Sep 21, 2021 at 18:02
• @immediate_blood: normal sort of makes sense but, more precisely, do you mean that the log price follows a geometric brownian motion ? Sep 21, 2021 at 21:06