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Question 19 - quadratic probility problem

  • Thread starter thomas49th
  • Start date
656
0
img021.jpg


a) (i) (x-14)(2x-7)
(ii) x = 14 or x = 3.5

b)
i) [tex]\frac{7}{n+7}[/tex]
ii) Take n to be 8

[tex]\frac{7}{8+7}[/tex]
[tex]\frac{7}{15}[/tex] that DOESN'T round down to [tex]\frac{2}{5}[/tex]

Is that all correct so far?
If so I will post the next (really hard) question)...

Thanks
 

HallsofIvy

Science Advisor
Homework Helper
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Why take n to be 8? Nothing is said about any value for n.
If 7/(n+7)= 2/5, SOLVE for n. What happens?
 
656
0
[tex]\frac{7}{n+7}[/tex] = [tex]\frac{2}{5}[/tex]

cross multiply!

[tex]\frac{35}{2n+14}[/tex]

now where?
 
206
0
Um...you're leaving out the equality part of the equation. Solve for n.
 
206
0
Alternatively, you can note that you have 35/(2n + 14) = 1. 35 is odd. 2n + 14 is even. Strange, isn't it?
 
656
0
uh? How does that equal 1?
 
206
0
Look, you're essentially supposed to say, "Suppose Bill is right. Suppose 7/(n + 7) = 2/5. Then such and such would follow." Why would the conclusion be a problem?
 
656
0
[tex]\frac{35}{2n+14}[/tex] = 1

now I need to get N on it's own (don't know how- plz show). But im guessing that n is greater than 3/5 so it CANT be right?
 

danago

Gold Member
1,122
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Multiply both sides of the equation by 2n + 14, so you end up with:

35 = 2n + 14
 
656
0
silly me....

2n = 35-14
n = 10.5
EDIT: Which as a fraction is 10/1/2 which DOESN'T equal 2/5...am i right yet. I doubt thats right...
 
206
0
You're getting there. What's the problem with n being 10.5. Look at your original assumptions. What are you tacitly assuming about the original n balls?
 
656
0
you cant have 1/2 a ball... :bugeye:

am I right or am I right
 
Last edited:
656
0
Okay here is the rest of the question

img022.jpg
 

Hootenanny

Staff Emeritus
Science Advisor
Gold Member
9,598
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Okay, what are your ideas on part (c)?
 
656
0
(probability of taking white x probability of taking yellow) + (probability of taking yellow x probality of taking white)

Is that somthing to go from?

Thanks
 

Hootenanny

Staff Emeritus
Science Advisor
Gold Member
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Yes, I'd go with that.
 
656
0
I got it down now to [tex]\frac{14n}{2n^{2}+28n+98}[/tex] = [tex]\frac{4}{9}[/tex]

is that right so far?

EDIT: is the question is says -28n but ive got +28n


EDIT 2: O no it must be this so far

[tex]\frac{14n}{n^{2}+14n+49}[/tex] = [tex]\frac{4}{9}[/tex]
 
656
0
got it!

[tex]\frac{4n^{2} + 56n + 196}{2}[/tex] = [tex]\frac{14n * 9}{2}[/tex]
[tex]\2n^{2} + 28n + 98[/tex] = [tex]68n[/tex]


[tex]\2n^{2} - 35n + 98 = 0[/tex]
 
656
0
and the answer to d must be 1/9
x must be 14 as you cant have 3.5 balls
 

Hootenanny

Staff Emeritus
Science Advisor
Gold Member
9,598
6
All sounds good to me, well done :approve:
 

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