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)?

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


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