# Binomial Distribution home work question

I attempted this problem on binomial distribution and I am getting an answer of 1/4 whereas the answer given in the text was 1/8. I used the formula for binomial distribution, Here goes the question:

Consider an urn in which 4 balls have been placed by the following scheme : A fair coin is tossed; if the coin falls head, a white ball is placed in the urn, and if the coin falls tail, a red ball is placed in urn. (i)What is the probability that the urn will contain exactly 3 white balls ?

ATTEMPT P(X = 3) = nCr * (1/2)^r * (1/2)^(n-r)

n = 4 ,r = 3, yields a result of 1/4. Regards

$\binom 43 2^{-4}$ is correct way to get the answer, and that is $4^{-1}$.