# conditional probability/independence

• Mar 28th 2010, 05:38 AM
shawli
conditional probability/independence
A box contains 5 red, 3 black, and 2 white cubes. A cube is randomly drawn and has its colour noted. The cube is then replaced, together with 2 more of the same colour. A second cube is then drawn.

Given that the first cube selected was red, what is the probability that teh second cube selected is black?
• Mar 28th 2010, 06:02 AM
Sudharaka
Quote:

Originally Posted by shawli
A box contains 5 red, 3 black, and 2 white cubes. A cube is randomly drawn and has its colour noted. The cube is then replaced, together with 2 more of the same colour. A second cube is then drawn.

Given that the first cube selected was red, what is the probability that teh second cube selected is black?

Dear shawli,

If the first cube selected is a red one then there are, 7 red cubes, 3 black cubes and 2 white cubes (altogether 12 cubes).

Therefore the probability the second cube is a black one= $\frac{3}{12}=\frac{1}{4}=0.25$