Probability of picking one of several balls of one of two colors

[sara_87]

An bag contains four yellow balls and two green balls. Find the probability of getting a yellow
ball if you. . .
(a) . . . randomly choose a ball from the urn.
(b) . . . randomly choose and discard two balls and then choose a third.

i know how to do part a..it's just 4/6 = 2/3
but for part b, i can't do it because there will be three cases: discarding 2 yellow, discarding 2 green or discarding 1 yellow and 1 green but i should only get 1 answer
?

 

[mathman]

For part (b) you will find it easier to do if you assume that you DON'T look at the 2 balls you discarded. In this way you will see the answer is still 2/3.

 

[sara_87]

but how did you get 2/3 ?
i tried to work it but couldn't know how??

 

[mathman]

If you don't know what the 2 discarded balls are, then the reasoning for (b) is identical to the reasoning for (a).