Hey guys,

I am wondering if any one you guys can explian to me a very basic concept in probablity?

A bag of 10 marbles contains 3 red marbles and 7 blue marbles. If two marbles are selected at random, what is the probability that at least one marble is blue?

A. 21/50

B. 3/13

C. 47/50

D. 14/15

E. 1/5

Method used to solve this is by determining the probability that no marbles are blue, and then by subtracting that probability from 1 and determining that probability that at least one marble is blue.

WHAT IS WRONG IF -

you do this in two steps like this -

1. atleast 1 marble is blue means either 1 marble is blue and the other is red or both of them are blue.

2. so, determine the probablity of in both cases and then add them as per the additive law.

I will really appreciate if someone were to throw some light on this.

Thanks.

_________________

Cheers, Rahul.