
TREE DIAGRAMS: 
Example: If it rains on a given day, the probability that it rains the next day is 1/3. If it does not rain on a given 
day, the probability that it rains the next day is 1/6. The probability that it will rain tomorrow is 1/5. What is the 
probability that it will rain the day after tomorrow? Draw a tree diagram of all the possibilities to determine the 
answer.  
 
Answer: 
Draw the first level of the tree diagram  
Before we  can  determine  what  happens  on the  day  after  tomorrow,  we  first  have  to  determine what 
might happen tomorrow. We are told that there is a 1/5 probability that it will rain tomorrow. Here is 
how to represent this information using a tree diagram:  
 
 
 
Draw the second level of the tree diagram  
We are also told that if  it does rain on one day, there is a 1/3  probability that it will also rain  on  the 
following day. On the other hand, if it does not rain on one day, there is only a 1/6  probability that it will 
also rain on the following day. Using this information we complete the tree diagram: