Answer:
A/b
B/d
Step-by-step explanation:
 
        
             
        
        
        
So chance to get a black pen: 4/10=2/5 
Chance to get a red pen: 1/10
Chance for the two happened at the same time
2/5 * 1/10 = 2/50=1/25
I think... I’m not very sure though
        
             
        
        
        
Answer:
Use the value for the variable to solve for the other unknowns.  Substitute the value for the variable into the expression for the number of seats on the middle level.  Substitute the value for the variable into the expression for the number of seats on the lower level.
 
        
                    
             
        
        
        
Root root 20^2 - 4^2
X = 2