P(One head exactly) \(=\dfrac{1}{2} \times \dfrac{1}{2} \times \dfrac{1}{2} \times \dfrac{3}{8} \)
The first \(\dfrac{1}{2} \) is the probability of getting a head. The other two halfs are the probability of NOT getting a head. The 3 is because the head could appear on either the first OR second OR third throw.