# CFA Mock Morning session Quants Q25

I have been scratching my head for this question but it doesn’t make any sense to me even after reading the explanation. Although I don’t know what should be the solution for this question but I am not convinced the explanation either. Can any one of you please explain how P(A or B) and P(A or B or AB) are the same probabilities?

I second. This one threw me off. Thoughts?

Not only this, I mean there are so many questions in which I got tricked, mostly because they weren’t clear enough. In fact, explanations have lot of gaps. I never had any problems schweser although the question are tough but I could never find a reason to doubt the questions or the solution provided.

hahaha, this one just gave me a stroke. I was like WTF !!! still now, don’t get how they dealt with the AB part man

Its most likely another typo… I mean we know how P(AB) is computed and we know how P(AorB) is computed… and I don’t see any logical connection in P(A or B or AB).

a typo ?.. the choices include the proba with AB, and the explanation is definitely smthg else…really bizar

Miss*Yiota Wrote: ------------------------------------------------------- > a typo ?.. the choices include the proba with AB, > and the explanation is definitely smthg > else…really bizar Quant is not my strongest, but I have no idea how to relate these two options… Its sometimes sooo frustrating not having some type of live “professional” you can ask these questions…

think about it like a Venn diagram – P(A) and P(B) are the two overlapping circles. If you just add together A and B, you will be ‘double counting’ the part where they overlap, P(AB). So you have to add them together and then subtract P(AB) to get the answer. I agree the wording is tricky and designed to throw us off…but the thing to remember is that P(A) + P(B) - P(AB) covers the possibility of A, or B, or both (again, think of the venn diagram). hope that helps.

Kiakaha Wrote: ------------------------------------------------------- > think about it like a Venn diagram – P(A) and > P(B) are the two overlapping circles. If you just > add together A and B, you will be ‘double > counting’ the part where they overlap, P(AB). > > So you have to add them together and then subtract > P(AB) to get the answer. > > I agree the wording is tricky and designed to > throw us off…but the thing to remember is that > P(A) + P(B) - P(AB) covers the possibility of A, > or B, or both (again, think of the venn diagram). > > hope that helps. hmm makes sense…Thanks!

Kiakaha Wrote: ------------------------------------------------------- > think about it like a Venn diagram – P(A) and > P(B) are the two overlapping circles. If you just > add together A and B, you will be ‘double > counting’ the part where they overlap, P(AB). > > So you have to add them together and then subtract > P(AB) to get the answer. > > I agree the wording is tricky and designed to > throw us off…but the thing to remember is that > P(A) + P(B) - P(AB) covers the possibility of A, > or B, or both (again, think of the venn diagram). > > hope that helps. This is perfect! Great answer. I totally forgot about the overlapping and then the potential for double counting… pg 444 for all those confirming. Much appreciated.