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# Help: S2 Binomial Distribution Question watch

1. This question is Q7 on P17 of the Edexcel S2 Pearson Company textbook:

A 6 sided die is biased. When the die is thrown the number 5 is twice as likely to appear as any other number. All the other faces are equally likely to appear. The die is thrown repeatedly. Find the probability that

a) the first 5 will occur on the sixth throw
b) in the first 7 throws there will be exactly three 5s.

Help much appreciated. Thanks!
2. Have you worked out the prob of rolling a 5?
3. (Original post by vc94)
Have you worked out the prob of rolling a 5?
No, that is what I'm not sure how to get.

I thought maybe it was 2/7.
4. yes it has to be, so that sum of all probabilities is 1, i.e. 2/7 + 1/7 +...1/7 = 1

a) is not binomial. want prob of "not 5" ...5 times in a row AND then prob of a 5 (on the sixth roll).
b) is binomial, n =7, p=2/7
5. (Original post by vc94)
yes it has to be, so that sum of all probabilities is 1, i.e. 2/7 + 1/7 +...1/7 = 1

a) is not binomial. want prob of "not 5" ...5 times in a row AND then prob of a 5 (on the sixth roll).
b) is binomial, n =7, p=2/7
So for part(a)... is it

(1/7)^5 * (2/7)
Because it doesn't seem to give me the right answer?

And thanks, I've now got part (b). Just unsure on (a).
6. (Original post by Lady_Diamond)
So for part(a)... is it

(1/7)^5 * (2/7)
Because it doesn't seem to give me the right answer?

And thanks, I've now got part (b). Just unsure on (a).
You have the right idea, but the probability of "not5" does not equal 1/7. Have another go.
7. Aha! Just got it! Thank you!
8. (Original post by Lady_Diamond)
Aha! Just got it! Thank you!

PS: It would help if you said how far you'd got and what you've been able to do, and what's causing the problem on your first post (as per forum guidelines); saves a lot of time.

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