Question

Consider a coin that a head is twice more likely to occur than a tail. Find the variance of number of heads when the coin is tossed three times.

Answer 1

0.667

Explanation:

Given that :

Head is twice more likely to occur than a tail.

P(head) = p(H) = 2/3

P(tail) = p(T) = 1/3

If coin is tossed 3 times :

P(H = 0) :

1st toss = tail ;2nd toss = tail ; 3rd toss =. Tail

(1/3) * (1/3) * (1/3) = 1 / 27

P(H= 1)

HTT, THT, TTH

(2/3 * 1/3 * 1/3) + (1/3 * 2/3 * 1/3) + (1/3 * 1/3 *2/3)

= 2/27 + 2 /27 + 2/27

= 6/27 = 2/9

P(H=2)

HHT, HTH, THH

(2/3 * 2/3 * 1/3) + (2/3 * 1/3 * 2/3) + (1/3 * 2/3 * 2/3)

= 4/27 + 4/27 + 4/27

= 12 / 27

= 4 /9

P(H = 3)

HHH

(2/3 * 2/3 * 2/3)

= 8 / 27

X ___ 0 ___ 1 ____ 2 ____ 3

P(x) _ 1/27 _ 2/9 __ 4/9 __ 8/27

E(X) = (0 * (1/27)) + (1 * (2/9)) + (2 * (4/9)) + (3 * (8/27)) = 2

E(X) = 2

Var(X) = Σx²p(x) - E(X)²

= (0^2 * (1/27)) + (1^2 * (2/9)) + (2^2 * (4/9)) + (3^2 * (8/27)) - 2^2

= 4.667 - 4

= 0.667

Hence, Var(x) = 0.667