3.68. If the joint probability density of X, Y, and Z is given by
\begin{equation*}
f(x, y, z) = \begin{cases}
\frac{1}{3}(2x + 3y + z) & \text{for } 0 < x < 1, 0 < y < 1, 0 < z < 1 \\
0 & \text{elsewhere}
\end{cases}
\end{equation*}
find
(a) $P(X = \frac{1}{2}, Y = \frac{1}{2}, Z = \frac{1}{2})$;
(b) $P(X < \frac{1}{2}, Y < \frac{1}{2}, Z < \frac{1}{2})$.