Question

6.5% of people in the U.S. Have A- blood type. You randomly select 6 Americans and ask them if their blood type is A-. a) Find the probability that all 6 are type A-. b) Find the probability that at most 4 of them are type A-. c) Find the mean and standard deviation.

Answer 1

Answer:

a) 7.54189*10^-8

b) 0.99999999

c) E ( X ) = 0.39 , s ( X ) = 0.604

Step-by-step explanation:

Solution:-

- We will assume the proportion of people with A- blood group is independent and remains constant for a fairly small sample of n = 6 Americans selected at random.

- We will denote a random variable X = The number of americans out of 6 that have blood group type A-.

- The random variable is assumed to follow binomial distribution.

- The probability of success is the proportion of people in U.S that have blood group type A-, p = 0.065:

                        X ~ Bin ( 6 , 0.065 )

Where, r represents the number of americans out of selected 6 have blood group A-. The pmf of a binomial variate is given as:

                       P ( X = r ) = nCr * ( p ) ^r * ( 1 - p ) ^ ( n - r )

a) Find the probability that all 6 are type A-

- We will pmf given above and set r = 6. And evaluate the resulting probability:

                     P ( X = 6 ) = 6C6 * ( p )^6 * ( 1  - p ) ^ ( 0 )

                                      = p^6

                                      = ( 0.065 )^6  

                                      = 7.54189*10^-8

b) Find the probability that at most 4 of them are type A-

- We will pmf given above and evaluate the following expression:

                     P ( X ≤ 4 ) = 1 - P ( X = 5 ) - P ( X = 6 )

                     P ( X ≤ 4 ) = 1 - 6C5 * ( p )^5 * ( 1  - p ) ^ ( 1 ) - p^6

                                      = 1 - 6*(0.065)^5 ( 0.935 ) - 0.065^6

                                      = 1 - 6.50923*10^-8 - 7.54189*10^-8        

                                      = 0.99999999          

c) Find the mean and standard deviation.

- The mean E ( X ) of the defined random variable distributed binomially is given by:

                     E ( X ) = n*p = 6*0.065 = 0.39 people

- The mean s( X ) of the defined random variable distributed binomially is given by:

   

                    s ( X ) = √n*p*q = √(6*0.065*0.935) = 0.604