求PREP-DS-2-152

jesuisdesole

A certain jar contains only b black marbles, w white marbles, and r 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

 

 

答案 A

 

实在不知道怎么算这玩意

 

崩溃ING.......

 

请高手指教

多谢多谢

stpearl

Pr(red)>r(white) we get

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

condition1: we get

r/(total-r)>w/(total-w) this is positive relation, so r>w

this ratio is odds ratio you can wiki this

condition 2: b>w+r insuffi

QED