Answer
$\frac{1}{8}$, see below.
Work Step by Step
The sample space should contain 32 possible outcomes in the form of $Mxy$ where $M=$Forward or Backward, $x=1,2,3,4$, $y=1,2,3,4$.
The final probability is the ratio of all the favorable outcomes over 32, or we can find it by using the product of each individual probabilities, we have
$P=(\frac{1}{2})(\frac{2}{4})(\frac{2}{4})=\frac{1}{8}$
