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secondary 4 | A Maths
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Candice lim
Candice Lim

secondary 4 chevron_right A Maths chevron_right Singapore

Good morning,
Could you help to explain the solution for this question to me please? Thanks a lot!

Date Posted: 3 years ago
Views: 657
Eric Nicholas K
Eric Nicholas K
3 years ago
Good afternoon Candice! The idea for this one should not be seen in Sec 4, because the simplification of this is probably going to involve the idea of geometric series from JC level maths.
Eric Nicholas K
Eric Nicholas K
3 years ago
The tutor has given a rough idea for the “exactly n tries” solution, and this idea alone is within the Sec 4 syllabus for probability, but then we need to simplify the result when it’s “at least n tries” and this one involves JC knowledge.
Candice lim
Candice Lim
3 years ago
Thanks for the explanation Mr Eric :)
I found this question in one of the other school's Prelim paper that is meant for their Sec 4 students last year.
Would you be able to spare some time to explain to me on this question? I would still very much like to understand about the solution. Thank you so much.

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Eric Nicholas K
Eric Nicholas K's answer
5997 answers (Tutor Details)
Good morning Candice! This is an alternative approach which is definitely more suitable for a Sec 4 student.
Candice lim
Candice Lim
3 years ago
Ohh…..thank you so much Mr Eric :)
I was having a hard time trying to figure out the previous working. I am now having my HBL. Once I am done with the zoom meetings, I will review your advised solution immediately.
I need to let you know that I really appreciate your continuous effort in helping me searching for an easier to understood solution.
Once again thank you very much!

I hope you have a great day ahead and pls take care :D
Candice lim
Candice Lim
3 years ago
Good afternoon Mr Eric :)
Yes...I can fully understand this question now. Thanks alot for your clear explanation!
Eric Nicholas K
Eric Nicholas K
3 years ago
Good evening Candice! I found an even easier idea for this question while I was working on a similar question with my student a while ago. The fact that it takes at least n draws to get an 8 means that no matter what happens, the first (n - 1) draws must have not been an 8.

Regardless of what happens later, everything which happens after the (n - 1) draws can be classified as taking “at least n draws”. You can verify this with numbers and tree diagrams.

As such, the priority here is to focus only on the first (n - 1) tries, each with probability of 7/8 so we have (7/8)^(n - 1).
Candice lim
Candice Lim
3 years ago
Good evening Mr Eric.
Thank you so much for coming back to me. I am very grateful that you have been looking out for a better way to explain this question to me :)
Yes, that is also how I understood it :D
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Danny Low
Danny Low's answer
1017 answers (Tutor Details)
Hope this helps
Candice lim
Candice Lim
3 years ago
Thanks for your advice, Mr Low.
The question asks for at least n draws. Your advised solution is for exactly n draws. Could you help to review the question again and advise me the solution for at least n draws to obtain an '8'?
Many thanks!
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Eric Nicholas K
Eric Nicholas K's answer
5997 answers (Tutor Details)
Good evening Candice! This forms the first part of my workings which I am still writing (but I have to erase first due to lack of board space).
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Eric Nicholas K
Eric Nicholas K's answer
5997 answers (Tutor Details)
Good evening Candice! This forms the second part of my workings. There might be an easier method out there which does not require the use of geometric series, otherwise this question should not appear inside this paper at all. Unless it’s an IP school or it’s an old syllabus...
Candice lim
Candice Lim
3 years ago
Good evening and thank you so much Mr Eric. Yes, this is the right answer :)