In essence, the tree method is a casework approach and very intuitive. In a probability tree:
- All nodes on a branch denote a series of consecutive and indepedent events.
- Different branches denote mutually exclusive events
To find the answer, we work from leaves towards root by:
- Multiplying probabilities of all the nodes along each branch
- Adding probabilities of all the child nodes of the same node
The tutorial video gives a visualized example of the above mentioned rules.
A bug crawls from $A$ along a grid. It never goes backward, it crawls towards all the other possible directions with equal probability. For example: The questions are, from $A$:(2732) 