prob question (1 Viewer)

[didi]

from the 1988 hsc paper, Q9(b)ii:
"A die whose faces are numbered 1, 2, 3, 4, 5, 6 is tossed twice. The sum S of the numbers which appear uppermost on the die is calculated...
It is known that a 4 appears on the die at least once in the two throws. Find the probability that S is greater than 8"

The answer they gave was 4/11. I don't understand how they got this. please help!!

didi

 

[mastermind]

Originally posted by didi
from the 1988 hsc paper, Q9(b)ii:
"A die whose faces are numbered 1, 2, 3, 4, 5, 6 is tossed twice. The sum S of the numbers which appear uppermost on the die is calculated...
It is known that a 4 appears on the die at least once in the two throws. Find the probability that S is greater than 8"

The answer they gave was 4/11. I don't understand how they got this. please help!!

didi

For there to be at least one 4, the sample space of the two numbers added S is:

1,4
2,4
3,4
4,4
5,4
6,4
4,1
4,2
4,3
4,4
4,5
4,6

which is 12 - but 4,4 is repeated so the sample space becomes 11

for the two numbers to be > 8, the possibilites are:

5,4
6,4
4,5
4,6

therefore, answer is 4/11

 

[didi]

oh i see...
i bet if i get a q like that in the hsc i'll be sitting there for ages trying to work it out. aarrgh!! i'm so dumb!

thnx 4 that.

didi

 

[mastermind]

no prob, didi...

if u keep practising them, you'll get the hang of it eventually...

 

[Minai]

with those die probability questions, its easier to jus draw box diagrams (a grid which shows all the combinations)
it helps to understand the scope of the problem your given

 

[mastermind]

Yeah MinAi, I agree. I did that for this question, but had to write out all the combinations cause i cant put a table in here (can I??)

So, that is a good suggestion, it really does help for this type of problem.

 

[Lazarus]

You can use HTML codes to insert tables.

 

[mastermind]

Originally posted by Lazarus
You can use HTML codes to insert tables.

Oh, ok. Thanks Lazarus.

Still, I would have typed out the same amount of info anyway. But it would have been more helpful to do it that way.

 

[spice girl]

Man, how complicated can chucking two dice be?

You can probably do it in your head.

Remember, if chucking 2 dice, one number is known, then sample space is 11.

 

[Minai]

sorry spice...but we here dont do 4U or have Ruse-capacity-knowledge (well, some of us anyway..)