# Probability

#### ShannonL

##### New Member
Hello to everyone...

I'm very new to stats and have worked through my assignments but am not sure I'm on the right track.

The question is: Two events, A and B are equally likely. The chance that either A or B happens on a given trial of an experiemtn is 0.70, while the chance that they both happen on a given trial is only 0.40 What is the probability that event A happens?

I know this is probably pretty basic for you all and I am not looking for help with homework...I'm just really new to stats and am finding some of the concepts pretty difficult.

Thanks alot...

#### quark

Hi Shannon,

Welcome to the forum.

You can use the equation P(A or B) = P(A)+P(B)-P(A and B), since P(A or B) and P(A and B) are given, and P(A)=P(B), it shouldn't be difficult to solve for P(A). Let me know if you have further questions.

#### ShannonL

##### New Member
Thanks very much for your reply and please bear with me. Am I correct then, in determining that A would be the result of P(A or B) - P(A and B) or .30?

I had determined that I should use the addition equation - as you noted below - and although I know this is relatively simple for the experienced person, I'm having a bit of a time with it.

Thanks again and I appreciate your help...I'm hoping to be able to continue on my own once I'm able to wrap my brain around this concept.

#### quark

Shannon,

Just substitute the numbers into the formula:

0.70 = P(A)+P(B)-0.40

0.70 = P(A)+P(A)-0.40 since P(A)=P(B),

and solve for P(A).

#### ShannonL

##### New Member
Thanks alot for your help...much appreciated! Shannon