(a)
$\begin{align*} &(a) \\ &\text{Required probability} = \frac{0.5*0.5}{0.5*0.5+0.5*1} = \frac{1*0.5^2}{1*0.5^2+0.5} = \frac{1}{3} \\ \\ \hline \\ \end{align*}$
(b)
$\begin{align*} &(b) \\ &\text{Required probability} \\ &=\frac{0.5*0.5*0.5 + 0.5*0.5*0.5 }{0.5*0.5*0.5+0.5*0.5*0.5+0.5*1*1} \\ &= \frac{2*0.5^3}{2*0.5^3+0.5} \\ &= \frac{0.5^2}{0.5^2+0.5} \\ &= \frac{1}{3} \\ \\ \end{align*}$
(c)
$\begin{align*} &(b) \\ &\text{Required probability} \\ &=\frac{4*0.5*0.5*0.5*0.5}{4*0.5*0.5*0.5*0.5+0.5*1*1*1} \\ &= \frac{4*0.5^4}{4*0.5^4+0.5} \\ &= \frac{0.5^2}{0.5^2+0.5} \\ &= \frac{1}{3} \\ \\ \end{align*}$.