Geometry

Question Analysis  
This question applies the addition rule for mutually exclusive events (since a marble cannot be both red and blue).  

Key Concept Explanation  
For mutually exclusive events, P(A or B) = P(A) + P(B).  

Step-by-Step Solution  
1. Total marbles = 5 (red) + 3 (blue) + 2 (green) = 10.  
2. P(Red) = 5/10 = 0.5.  
3. P(Blue) = 3/10 = 0.3.  
4. P(Red or Blue) = 0.5 + 0.3 = 0.8.