# Homework Help: Probability of throwing dice

1. Jul 15, 2014

### desmond iking

1. The problem statement, all variables and given/known data

A pair of dice are thrown. If a die show number 6 , what is the probability of (a)another die show number 2? (b) total score for both number greater than 8 ?

for part (a) , my working is 1/6 x 1/6 = 1/36

but actual ans is 2/11 for part (b), the ans is 7/11

2. Relevant equations

3. The attempt at a solution

2. Jul 15, 2014

### LCKurtz

Write down all the outcome pairs where at least one die shows a 6. How many such pairs do you have? How many of those pairs also include a 2?

3. Jul 15, 2014

### desmond iking

(1,6) , (2,6) , (3,6) , (4,6) , (5,6) , (6,6)

one contain (2,6)

4. Jul 15, 2014

### LCKurtz

5. Jul 15, 2014

### desmond iking

hi, i got the solution now. but i just cant understand why i cant use the probability tree to get the ans as the photo below?

#### Attached Files:

• ###### IMG_20140716_123025[1].jpg
File size:
34.1 KB
Views:
119
6. Jul 15, 2014

### vela

Staff Emeritus
You drew a tree, but you haven't shown getting an answer to the question using it. What answer do you think the tree is leading you to?

7. Jul 15, 2014

### haruspex

Is that the exact wording? If so, it's a terrible question. As LCKurtz presumed, it almost certainly intended "if at least one die shows a 6", but that's not what it says.
It all hinges on the basis for selecting the die showing a six. The wording suggests that the observer picked one die at random and saw that it was a six. That will not give 2/11 as the answer.