mikemcgarry wrote:
pacifist85 wrote:
A box contains red balls, white balls, and yellow balls. If a ball is randomly selected, what is the probability that it will NOT be yellow?
(1) The probability that the ball will be red is 1/2.
(2) The probability that the ball will be yellow is 1/3.
*Nice one - I think. It is not difficult, but has a tiny trick if you are not careful.
Dear
pacifist85,
This appears to be a very straightforward test of the
complement ruleP(not A) = 1 - P(A)Consider "
the probability of that A happens" and "
the probability that A doesn't happen" --- having either one automatically gives us the other. See:
https://magoosh.com/gmat/2012/gmat-math- ... -question/Is the
complement rule the "trick" you had in mind?
Mike
Hi Mike,
Indeed the "1minus the opposite" was mainly the trick I was referring to.
But also, I think that you can get confused because the problem does not actually give you the number of the balls - so we don't know how many they are. So, initially, one might think that, oh, none of the 2 data gives me the number of balls, so I cannot possibly answer this question. Then, being in a hurry anyway, you may disregard the second data point and move on.
Also, the first data point gives you 1/2, which is 50% and the second one refers to the second of the 3 colours. This may incorrectly lead you to assume that you know how many half of the balls are. The rest is the other 50%. However, we have three colours here and the data points only refer to 2. Your poor, tired eye, having seen the 50% and reading clues about 2 of the three colours, might forget that the question stem actually mentioned 3 colours.
Or is it only me...