Step-by-step explanation:
The explanation and the graphical representation is attached
 
        
             
        
        
        
Answer:

Step-by-step explanation:
 
        
             
        
        
        
Answer:
The speed of the boat in still water is 10 miles / hour
Step-by-step explanation:
The trip there, and the trip back are the same -- 30 miles
Let the rate of the boat = x
Let the rate of the water = y
30 = 2*(x + y)
30 = 6*(x - y)
Divide the top equation by 2
Divide the bottom equation by 6
15 = x + y
<u>5  = x - y   </u>            Add the equations together.
20 = 2x                 Divide by 2
20/2 = x
x = 10