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In a certain population, 15% of the people have Rh-negative blood. A blood bank serving this population receives 100 blood donors on a particular day.

(a) What is the probability that 15 to 20 (inclusive) of the donors are Rh-negative?

I got an answer of 0.4939 by subtracting the z scores of p(z<1.54) and p(z<-0.14). I also rounded 15 and 20 to 14.5 and 20.5 when inserting the values in the z-score formula.

My mean I got was 100 *0.15 = 15. And my standard dev is 3.5707 from the square root of npq.

I was wondering if someone could confirm my answer?

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    $\begingroup$ Does your problem specify that you need to use the normal approximation to the binomial distribution? You can also look up /calculate the values for a binomial distribution to double check your work. $\endgroup$
    – M Waz
    Commented Jul 29, 2019 at 19:24

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Using the exact Binomial distribution, the result is 0.4764. Using the Normal Approximation with continuity correction, which is what you are doing, the result is 0.4939.

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