From a bag containing 12 blue balls, y yellow ball, and no ther balls, one ball will be removed at random. If the probability of selecting a blue ball is less than 2/5, what is the least number of yellow balls that must be in the bag?
1-17
2-18
3-19
4-20
5-21
I keep getting 18, OA is 19. Can someone please explain and/or tell me where I am going wrong? Thanks guys
Gmatprep probability
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Let's assume that the probability is EXACTLY 2/5 that you'll pull out a blue ball. So, if you have 12 blue balls,maolivie wrote:From a bag containing 12 blue balls, y yellow ball, and no ther balls, one ball will be removed at random. If the probability of selecting a blue ball is less than 2/5, what is the least number of yellow balls that must be in the bag?
1-17
2-18
3-19
4-20
5-21
I keep getting 18, OA is 19. Can someone please explain and/or tell me where I am going wrong? Thanks guys
12/x = 2/5
2x = 60
x = 30 => which means you need 30 balls total.
So,
(total balls - blue balls = yellow balls)
30 - 12 = 18
So you have 18 yellow balls. Looks like you got up to here all okay.
However, this means that you need 18 balls if you need the probability of pulling out a blue ball to be exactly 2/5. To make the probability less than 2/5, you would need to add 1 more yellow ball, so answer = 19.
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thanks again termis. This must be why my weakpoint for the GMAT in the verbal section is CR - I don't read carefully enough. How can I fix this?
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I'm no better, really. I've been doing sets of problems on a regular basis for 4 months now, so I do catch myself more often before I do something silly, but I still do it once in awhile. When I see the solution, it kills me that I overlooked something so simple, but hey, you and I are definitely not alone.maolivie wrote:thanks again termis. This must be why my weakpoint for the GMAT in the verbal section is CR - I don't read carefully enough. How can I fix this?
After you select an answer, just read the question quickly and see if it makes sense. But above all, just keep on hammering away at different questions on a regular basis, and you should get the hang of it over time.
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Here's How I did it :
Let yellow balls in the bag be Y
so,its given that
12/12+Y < 2/5
12 is the no. of blue balls, 12+Y is total no. of possible outcomes i.e. total no. of balls (blue+yellow)
Simplifying, solving the inequality
2Y + 24 > 60
Y + 12 > 30
Y > 18,
So,minimum value Y can take is 19
Let yellow balls in the bag be Y
so,its given that
12/12+Y < 2/5
12 is the no. of blue balls, 12+Y is total no. of possible outcomes i.e. total no. of balls (blue+yellow)
Simplifying, solving the inequality
2Y + 24 > 60
Y + 12 > 30
Y > 18,
So,minimum value Y can take is 19