stolyar wrote:

#1

1. Whats the probability that the fifth marble is red.

8/15*7/14*6/13*5/12(four are red)*4/11 (red again)

2. Whats the probability that the fifth marble is black.

8/15*7/14*6/13*5/12(four are red)*7/11 (black)

#2

A box contains 10 balls ,7 green, 3 yellow . You pick 5 one by one. Out of the first three, 2 green and 1 yellow.

Whats the probability that the remaining balls are a red and a yellow ?

P=0 there are no red balls in a box

Are you sure that the probability DOES NOT change even if we have knowledge of prior events.

This problem is different in that ...we already know that some event has occured..so wont that have an impact on the probability of a future event

We want to find out P( fifth is red / four are red )

i think your solution to # 1 will be correct ONLY when we want to know whats the probability of picking 5 red balls.

again for #2, i have the same doubt.

can you clarify please?

thanks

praetorian