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Assignment X4, Q4.5

A

ALEX_AK

Member
Hello,

Is it correct if I just find

P(X>=25)=(50C25)*0.4^24*0.6*25
= 0.0405

This is also the size of the test. Am I right? Isn't it faster doing it this way? Why must we use the Normal Approx as in the solution?
 
From your formula it looks like you're calculating P(X=25), but the question is asking for P(X>=25)
P(X>=25) = P(X=25) + P(X=26) + ... + P(X=50)

You could calculate this using the probability function, but you'd need to calculate and sum all the individual probabilities, so it's quicker to use the normal approximation.
 
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