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# Edexcel S2 watch

1. A S2 question need help with. candidate picks his answer to each question at random from list of 3 answers with only 1 being correct. a candidate answering 18 or more questions correctly passes the examination.

it is required that prob that candidate passes is less than 0.005. use a normal approximation to show that the paper should contain at most 31 questions...

im just getting confused by whole question...need help please asap...thanx
2. (Original post by nacero2000)
A S2 question need help with. candidate picks his answer to each question at random from list of 3 answers with only 1 being correct. a candidate answering 18 or more questions correctly passes the examination.

it is required that prob that candidate passes is less than 0.005. use a normal approximation to show that the paper should contain at most 31 questions...

im just getting confused by whole question...need help please asap...thanx
weird hour of the day to be doing S2 - i remember doing that somewhere...
here it is:ex 3b q 8b)?

P(X>17.5)<0.005 where Y~N(np, npq)
17.5-np/rt(npq) <-2.5758
(17.5-np)^2 < 6.6347(npq)
306.25-35np+(np)^2 <1.474n
solving for n

n=31.9 or 86.35 therefore:
n<31.9 ir n<86.35

n<31.9 satisfies both equations therefore as n is an integer, n =31 questions

not re-read the question - just typing up my solution

hope th is helps,
Phil

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