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.
