Probability of an event occurring at least once over n trials

[helpmeplzzz]

There is a 1 in 4 chance that on any given sexual encounter, Joe will contract an STD. If Joe has sex 4 times, what are the chances of him having an STD?

 

[pwsnafu]

Hint: what is the probability of not contracting after 4 times?

 

[helpmeplzzz]

1-(.75*.75*.75*.75) = my answer?

 

[HallsofIvy]

Looks good.

 

[helpmeplzzz]

So how would you do it for the probability of Joe contracting an STD twice out of those 4 occasions?

I think you would have to do a combination for this?

 

[Mark44]

As this is misplaced homework-type question, I am locking it. This thread was also posted in the Precalc section under Homework & Coursework.