S2 question thats driving me mad!

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#21
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okay heres how.....

P(x>17)=0.005
therefore P(x<(or equal to) 17) = 0.995 (1-0.005)

This approximates to N(Z<17.5) (Using continuity correction)

As this question follows a binomial distrbution with parameters (x,1/3)
Therefore this is approximatly equal to N(x/3, 2x/9)

Therefore Z< (17.5-x/3) / (square root (2x/9)

Probability = 0.995
Therefore Z is approximatly equal to 2.6

Therefore it can be said that:

(17.5-x/3) / (square root (2x/9) = 2.6

Solve to get x=31.8 or x= 86.7

Thus showing that the most x can be is 31!

Tell me if u still dont understand, and also reply so that i know uve got this!

Honestly i dont think edexcel will set a question like this, but if its nething like P3 then i guess its more than likely!
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I got up to (17.5-n/3) / (square root (2n/9) = 2.575
How on earth can you solve that?
There is no way you can get rid of the square root n, or is there?
Another way could be to take f(n) as the equation:
f(n)=17.5 - n/3 - 2.575(sqroot(2n/9))
Then plug in values f(30) and f(32) to show a change in sign using a bit of P2. Of course solving it properly as you have "appeared" to have done is the best way, but I couldnt get around simplifying the square root of n.
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When you have the equation you just get sqrroot of x on one side all on its own, then square both sides to make the x's neat, i.e. x^2, and x only.
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Rahul
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#23
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yeah i think the person asking knows, cos i already told him yday, but all posts get deleted (server change) or sumthing!!!
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