Answer:
$20
Step-by-step explanation:
15/3=5 which means that for every guest attending Carla's retirement party it must cost her $5 each. 
Therefore if 4 guests come to the party it must mean 4(5) which is 20.
Hence $20
 
        
                    
             
        
        
        
Step-by-step explanation:
9x+18 < 9-15
1)9-15= -6
9x+18 < -6
9x+18= -5
9x= (-5) - 18
9x= -23
x= 23/9=2 5/9
X=2,55556
 
        
             
        
        
        
Answer:
y=20x
Step-by-step explanation:
Since y represents the total cost y is the total. 20x because it is 20 dollars an hour and x after 20 because we do not know how many hours they will plan for lessons.