# Thread: How to reverse this calculation?

1. ## How to reverse this calculation?

Let's say the probability of X happening is 99%
The probability that X happens every time 10 times in a row is 0,99^10, right? So approximately 90%

so the numbers are:
A = 0,99
B = 10
C = 0,9

How do i find A, if I know B and C?

2. ## Re: How to reverse this calculation?

$A^{10}=0.9$

$A = (0.9)^{1/10} \approx 0.99$

3. ## Re: How to reverse this calculation?

Originally Posted by DanishGuy
Let's say the probability of X happening is 99%
The probability that X happens every time 10 times in a row is 0,99^10, right? So approximately 90%

so the numbers are:
A = 0,99
B = 10
C = 0,9

How do i find A, if I know B and C?
Let me rephrase your question (and notation) and see if I understand what you are asking. We have an event with probability $p$ of happening. We then do $n$ independent trials. Let $X$ be the random variable giving the number of successes in $n$ trials. Then $P(X=n) = p^n$. I think you are asking if you know $C=p^n$ and $n$, how can you find $A=p$. And the answer is $A=\sqrt[n]{p^n} = \sqrt[n]C$.