# Is this the right method?Watch

#1
I couldn't find a mark scheme for May 2006 OCR S1 so ill check on here.

The random variable Z has the distribution B-(n,0.27), find the smallest value of n such that

This is work so far

=

Therefore, P(Z = 0) < 0.05

So, 0.73^n < 0.05

n(Log0.73) < log0.05)

n > log0.05/log0.73

n > 9.51

n = 10

Im sure that for 3 marks this is way too much, if theres a simpler/correct method that would be much appreciated
0
8 years ago
#2
Apart from writing X when you should have written Z, and writing log(0.73)/log(0.05) instead of log(0.05)/log(0.73) [although it seems you calculated the latter], that seems right to me. You might be able to use tables to get to the answer, which could be why it's only worth 3 marks, but I'm not 100% sure about that.
#3
(Original post by nuodai)
Apart from writing X when you should have written Z, and writing log(0.73)/log(0.05) instead of log(0.05)/log(0.73) [although it seems you calculated the latter], that seems right to me. You might be able to use tables to get to the answer, which could be why it's only worth 3 marks, but I'm not 100% sure about that.
ah there we go, i was copying out of my book so i must have missed it out..cheers for that

thats what i thought, but the Probability only seems to go up in steps of 0.05 in my books after it goes from 0.1, like 0.15, 0.2, 0.25, 0.3

Fair enough, i was a bit confused when i had to bring in logs to solve a question that is halfway through the Statistics paper..
0
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