* probabilty 1-x trick by Lilly

[lillysonglove]

Please tell me why only A is correct and not B in the following questions.
There are 5 white balls and 3 black balls in a urn. two balls are drawn at random what is the probability that both the balls are not black?

a 1-(3C2/8C2) b 5C2/(8C2)

As if they are not black, they are white in this case, I get why a is correct, but I don't get why b is incorrect. thank you so much

[tim]

Where did this problem come from? I think the reason you're concerned about this one is because you are interpreting the constraint two different ways. The thing is, the way the question is worded it's not entirely clear which interpretation is correct, and one would yield A and the other would yield B as an answer. Rest assured that the GMAT would never give you a question that is so ambiguously worded, so you needn't worry about that problem. I can help you with the actual math of this problem if you can cite a source so we can track down whether the original is as problematic as you've indicated here.

[RonPurewal]

Please tell me why only A is correct and not B in the following questions.
There are 5 white balls and 3 black balls in a urn. two balls are drawn at random what is the probability that both the balls are not black?

a 1-(3C2/8C2) b 5C2/(8C2)

As if they are not black, they are white in this case, I get why a is correct, but I don't get why b is incorrect. thank you so much

hi,
is this a GMAT PREP problem?

* if not, this is the wrong folder; please post the problem in the appropriate folder, with a citation of the source. thanks.

* if this problem is actually from GMAT PREP, then it has definitely not been transcribed accurately. in this case, please enter the problem again, with all answer choices and exactly as originally written.

this is especially important in the problem at hand, because the problem here isn't a math issue; it's a wording issue.
(in short, "both ... are not black" is ambiguous; we don't know whether that means (i) it's false that both are black, or (ii) both are white.
choice a corresponds to interpretation (i), choice b to interpretation (ii).
a reliable source would not commit this sort of blunder -- not to mention other typographical errors, such as "a urn".)

please do one of the things listed above, depending on the source of the problem. if you don't, we'll have to delete the thread within a week or so.
thanks.