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# S3 Discrete Random Variable. Watch

1. Hi can someone please guide me through part (b) of this question:

I can't seem to understand how to do it?

Thanks
2. (Original post by singh224)
I can't seem to understand how to do it?
It's not a matter of understanding how to do it, but of writing things down and seeing what happens. Under what conditions would the total length of material be 120m?
3. (Original post by Smaug123)
It's not a matter of understanding how to do it, but of writing things down and seeing what happens. Under what conditions would the total length of material be 120m?
When there is order of 70m and 50m. Which is exactly 2 order but If I add the probability of those 2 I get 0.54. Is this right?
4. (Original post by singh224)
When there is order of 70m and 50m. Which is exactly 2 order but If I add the probability of those 2 I get 0.54. Is this right?
You've missed a possibility.
Also, by your logic, if I made three orders, there would be a 150% chance of getting 150m of material, so something's up with your method.
5. (Original post by Smaug123)
You've missed a possibility.
Also, by your logic, if I made three orders, there would be a 150% chance of getting 150m of material, so something's up with your method.
Please can you tell me what the right method is? I'm self teaching S2 so I can't see where I'm going wrong as the book doesn't show examples like these.
6. (Original post by singh224)
Please can you tell me what the right method is? I'm self teaching S2 so I can't see where I'm going wrong as the book doesn't show examples like these.
You've missed the possibility of 60 and 60.

7. (Original post by ExWunderkind)
You've missed the possibility of 60 and 60.

Thanks, so is the answer 0.0264?
8. (Original post by singh224)
Thanks, so is the answer 0.0264?
That's what I get, yes.
9. (Original post by ExWunderkind)
That's what I get, yes.
Thanks, the book is wrong I guess? it says 0.0464 :/
10. (Original post by singh224)
Thanks, so is the answer 0.0264?
Yep, that looks about right to me.
11. (Original post by singh224)
Thanks, the book is wrong I guess? it says 0.0464 :/
I hate it when that happens, it does make you second guess yourself. But I would say it's a typo, unless I too am making a mistake.

To get the answer the book has you'd have to use 60 and 50 which =/= 120.
12. (Original post by ExWunderkind)
I hate it when that happens, it does make you second guess yourself. But I would say it's a typo, unless I too am making a mistake.

To get the answer the book has you'd have to use 60 and 50 which =/= 120.
Thank you for the help, I guess I should not practice from the book instead go on paspapers because first time I did this method but the answer didn't match and it confused me lol.

Thanks again
13. (Original post by ExWunderkind)
I hate it when that happens, it does make you second guess yourself. But I would say it's a typo, unless I too am making a mistake.

To get the answer the book has you'd have to use 60 and 50 which =/= 120.
Unless they're doing (first order = 50 and second order = 70) + (first = 70 and second = 50) + (first = 60 and second = 60)
14. (Original post by singh224)
Thank you for the help, I guess I should not practice from the book instead go on paspapers because first time I did this method but the answer didn't match and it confused me lol.

Thanks again

(Original post by davros)
Unless they're doing (first order = 50 and second order = 70) + (first = 70 and second = 50) + (first = 60 and second = 60)
Yeah I think Davros has it. They are classing 50 + 70 and 70 + 50 as two separate combinations, which I guess makes sense.

And it does equal 0.0464.

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