Tree diagram branches not summing to 1
"The Branch Blunder"
The Mistake in Action
A spinner has probability 0.6 of landing on blue. Draw a tree diagram for spinning twice.
Wrong: First spin: P(Blue) = 0.6, P(Not Blue) = 0.6
Why It Happens
Students may copy the same probability for all branches or calculate the complement incorrectly.
The Fix
From any point, the branches must sum to 1.
If P(Blue) = 0.6, then P(Not Blue) = $1 - 0.6 = 0.4$
Check each "fork" in your tree:
- Does P(Blue) + P(Not Blue) = 1?
- $0.6 + 0.4 = 1$ ✓
This applies at every level of the tree!
Spot the Mistake
Can you identify where this student went wrong?
P(Blue) = 0.6 for each spin
P(Blue) = 0.6, P(Not Blue) = 0.6
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: