In item (e) you have "Given the fact that he was unsuccessful on the first day".

Therefore the term in the denominator shows the sum of two scenarios where Oscar could not find his dog.

The first being if he looks in forest A and could not find his dog i.e. 0.4 * 0.75.

Second being he looks in forest B and could not find his dog, in our case because we know dog is in A therefore he cannot find his dog in B. (0.6 * 1)

The two scenarios represented as Sa-->A-->F1c and Sa-->B-->F1c

Thank you for your query.