Gprep2-Probabilty Jar/Marbles

Question

why is C wrong?

gmatnub · Answer

some people may not want to open up another program just to view the question.

A

Sunny143 · Answer

We are asked,

r/b+w+r > w/b+w+r?

Which simplifies to 
rb+rw+r^2>wb+w^2+wr
rb+r^2>wb+w^2
r/b+w>w/b+r

Which is Statement 1

A

rpmodi · Answer

I solved it this way : condition 1 : r/(b+w)>w/(b+r) so (b+w)/r<(b+r)/w = (b+w)/r +1 <(b+r)/w +1 = (b+w+r)/r <(b+r+w)/w = r/(b+w+r) > w/(b+r+w) and so A is sufficient .

jusjmkol740 · Answer

But ain't both the statements say the same thing?

From Stmt 1) r.(b+r) > w.(b+w) => rb + r^2 > wb + w^2. 
 => rb + r^2 + rw > wb + w^2 + rw  
 So, r.(b+w+r) > w.(b+w+r). Thus, sufficient

From Stmt 2) b > r + w = b.(r-w) > (r+w).(r-w)  
 => rb - bw > r^2 - w^2 
 => r.(b+r) > w(b+w), which is actually Stmt1

So if Stmt1 is Suff, then Stmt2 shd also be Suff. Can anybody explain?

Bunuel · Answer

The question is  \frac{R}{R+B+W}>\frac{W}{R+B+W}  true? Or is  R>W  true?

(1)  \frac{R}{B+W} > \frac{W}{B+R}  -->  \frac{R}{B+W} +1> \frac{W}{B+R}+1  -->  \frac{R+B+W}{B+W}> \frac{W+B+R}{B+R}  -->  \frac{1}{B+W}> \frac{1}{B+R}  -->  B+R>B+W  -->  R>W . Sufficient.

(2)  B-W>R , not sufficient to determine whether  R>W  or not.

Answer: A.

As for your solution: you can not multiply inequality by  R-W , as you don't know whether this expression is positive or not, thus you don't know whether you should flip the sign of inequality or not. Actually when you multiply by  R-W  and not flipping the sign, you wrongly assume that  R-W>0 , or  R>W , but this is exactly what we are asked to determine.

Hope it's clear.

jusjmkol740 · Answer

Thanks. Missed it... :oops:

Here's an open request all - it would be very helpful to get a compilation of different types of solved probability problems. Is there a source?

tan888 · Answer

A

R/(B+W) > W(B+R)
R/(B+W)+1 > W(B+R)+1
R/(B+W)+R/R > W(B+R)+W/W
R/(B+W+R) > W/(B+R+W)

muslikcat · Answer

hmm, I took me more than 5 minutes to solve this question.

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