GMAT Prep <Test 2> Question 7

[puresillylily]

Question 7: 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

Answer: A --> condition A is sufficient to answer this question.

I need help to derive this answer!!

Thank you!!

[kaori110]

Question 7: 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

Answer: A --> condition A is sufficient to answer this question.

I need help to derive this answer!!

Thank you!!

設全部是A
第一個條件
r/(A-r) > w/(A-w)
-> rA-rw> Aw-wr
->r>w