Consider a random arrangement of 20 boys and 16 girls in a line. Let X be the number of boys with gi

Oberhangaps5z

Oberhangaps5z

Answered question

2022-05-12

Consider a random arrangement of 20 boys and 16 girls in a line. Let X be the number of boys with girls on both sides. Let Y be the number of girls with boys on both sides. Find E ( X + Y )
E ( ) denotes expectation. For a discrete random variable Z that takes the values z 1 , z 2 , , z n with the corresponding probabilities p 1 , p 2 , , p n the expectation of Z is defined as E ( Z ) = i = 1 n z i p i

Answer & Explanation

pradassas66b2d

pradassas66b2d

Beginner2022-05-13Added 11 answers

Use indicator functions and the Linearity of Expectation.
Let ( X i ) i { 2 , . . , 35 } be a sequence of indicators where X i is 0 unless 1 when the i t h member of the line is a boy and adjacent to a girl on either side.
Then X = i = 2 35 X i and E ( X i ) = P ( X 2 = 1 )
for i = 2 , , 35
So use Linearity of Expectation to find E ( X )
Do likewise for E ( Y )
Finally use Linearity of Expectation once more: E ( X + Y ) = E ( X ) + E ( Y )

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