# Probability of rolling 2 dice

• Jan 29th 2008, 09:52 AM
rmn
Probability of rolling 2 dice
Ok so there is this question I have been working on for over 2 hours including reading the whole chapter and going through my notes but it doesn't really sseem to make sense to me:
http://i207.photobucket.com/albums/b.../questiona.jpg
http://i207.photobucket.com/albums/b.../questionb.jpg

This is what i've done so far:
So P(B and D) I have 6/18 because thats how much of B there is in D right? or is it P(B) multiply P(D)? but still I don't understand when it asks P(B and D) = P(B) because isn't P(B) just 6/36? which isn't the same as 6/18?

If anyone could help me please :confused:
• Jan 29th 2008, 10:33 PM
CaptainBlack
Quote:

Originally Posted by rmn
Ok so there is this question I have been working on for over 2 hours including reading the whole chapter and going through my notes but it doesn't really sseem to make sense to me:
http://i207.photobucket.com/albums/b.../questiona.jpg

B denote the event of a sum of 7 on the two die, and D the event that the
sum of the two die is odd.

If B has occured then of necessity so has D as 7 is odd.

The even (B and D) means both events have occured, which always has the
same truth value as B, so in this case:

P(B and D)=P(B)

Alternativly draw a Venn diagam, then B is entirly enclosed withing D, and
the event (B and D) is represented by the area of overlap of B and D, which
in such a case is B.

Drawing a Venn diagram will also show why P(B or D)=P(D) when B is contained in
D.

RonL