Calculating certainty level

[George.W89]

It's been years since I've done probability questions, so I'm not sure if my logic is correct with the following scenario.

If something has a 3.5% chance of success, I want to know how many times should I repeat the the "roll" to be 95% certain of success?

This is the calculation I did:

(0.965)^x = 0.05
x=84.09

Thanks

 

[Dr.Peterson]

Presumably you mean that you want to have a 95% probability that at least one roll of N will succeed.

The way to handle this is that you want a 5% probability that all of the N rolls will be failures. This is because "all" is much easier to work with than "one or more". And that is exactly what you have done.

So you need at least 85 tries.

 

[George.W89]

Thanks for the response ?