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# S1 Edexcel Probability Distribution Urgent Help Required! watch

1. Hi everyone,
Part A = no problem. Part B, I filled in everything correctly but for the probability on the fourth attempt I did 0.4*0.4*0.4*0.6 = 0.0384. The correct answer is actually 0.064. I know I can easily add up my values for attempt 1,2 and 3 and subtract from one BUT my method should then give the same answer. I am very confused as to what I am doing incorrectly!
Thanks in advance, Creatures Of The Night.
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2. Okay so your issue is that there are 5 options:
1) correct on first attempt = 0.6
2) correct on second attempt = 0.4*0.6
3) correct on third attempt = 0.4*0.4*0.6
4) correct on 4th attempt = 0.4*0.4*0.4*0.4
5) incorrect on all attempts = 0.4*0.4*0.4*0.4
(4) + (5) = 0.064
3. (Original post by Darth_Narwhale)
Okay so your issue is that there are 5 options:
1) correct on first attempt = 0.6
2) correct on second attempt = 0.4*0.6
3) correct on third attempt = 0.4*0.4*0.6
4) correct on 4th attempt = 0.4*0.4*0.4*0.4
5) incorrect on all attempts = 0.4*0.4*0.4*0.4
(4) + (5) = 0.064
Ok, I understand my mistake. However, if that is the case, why did I not do that for all attempts? E.g. on second attempt it can be 0.4*0.6 = successful or 0.4*0.4 = unsuccessful, and then I add them up together...
Edit: I think I understand why. Because if it is unsuccessful on e.g. the second attempt, the probability of this unsuccess is taken into account on the 3rd attempt. But since the 4th attempt is the last one, we must take into account that the attempt has been unsuccessful. Is that right?
4. (Original post by Wolfram Alpha)
Edit: I think I understand why. Because if it is unsuccessful on e.g. the second attempt, the probability of this unsuccess is taken into account on the 3rd attempt. But since the 4th attempt is the last one, we must take into account that the attempt has been unsuccessful. Is that right?
This is correct, if you think about it in terms of a tree diagram, then the 5 options I explained are the only five outcomes (branches), so any extra calculation is incorrect

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