A fair coin is tossed four times What is the probability that exactly three tosses are heads?

n = number of trials = 4

r = number of success = 3

P(success) = P(H) = p = 1/2 (in 1 toss of a fair coin)

P(failure) = P(T) = q = 1/2

Use the formula nCr [p^(n-r)](q^r) :

(4C3)[(1/2)^1](1/2)^3

= (4C3)(1/2)^4

= 4 x 1/16

= 4/16

= 1/4