When anunfaircoin is tossed twice, the probability of getting one tails and one heads is 1/2. What is the probability of getting two heads and two tails if the coin is tossed 4 times?	Choose the correct choice:
A) 2 / 3
B) 1 / 3
C) 1 / 4
D) 1 / 6
E) 1 / 9

Rationale: let the probability of heads be x and the probability of tails be 1 - x . so , we are given that p ( one tails and one heads ) = 2 * x * ( 1 - x ) = 1 / 2 . p ( two heads and two tails ) = 4 ! / ( 2 ! 2 ! ) * x ^ 2 * ( 1 - x ) ^ 2 = 6 * ( x * ( 1 - x ) ) ^ 2 . we are multiplying by 4 ! / ( 2 ! 2 ! ) because hhtt scenario can occur in several ways : hhtt , htht , thht , . . . ( permutation of 4 letters hhtt where 2 t ' s and 2 h ' s are the same ) . since from 2 * x * ( 1 - x ) = 1 / 2 it follows that x * ( 1 - x ) = 1 / 4 , then 4 * ( x * ( 1 - x ) ) ^ 2 = 4 * ( 1 / 6 ) ^ 2 = 1 / 9 . answer : e .
Answer: E