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Jar of black, white and red marbles - probability

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Jar of black, white and red marbles - probability

by wally » Mon May 11, 2009 4:57 am
A certain jar contains only [i]b[/i] black marbles , [i]w[/i] white marbles, and [i]r[/i] red marbles. If one marble is to be chosen at random from the jar, is the probability that the marble chosen will be red greater than the probability that the marble chosen will be white?

1) r/(b+w) > w/(b+r)

2) b -w > r

ANSWER: [spoiler]A[/spoiler]
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by sacx » Mon May 11, 2009 5:32 am
r / (b+w+r) > w/(b+w+r)

we can cross multiply since b,r,w can only be positive

rb + wr + r^2 > wb + w^2 + wr
rb + r^2 > wb + w^2

Stmt I

r/(b+w) > w/(b+r)

rb + r^2 > wb + w^2

Suff

Stmt II

b-w > r
b > w+r

does not give any relationship between white and red balls

Insuff

Choose A
SACX
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by Ian Stewart » Mon May 11, 2009 2:08 pm
I suggested two non-algebraic ways to look at the problem here:

www.beatthegmat.com/marbles-probability-t32705.html

There's also an algebraic solution in that thread.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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