Answer: x=4/7
Step-by-step explanation:
15-20x=x+3
15-20x-15=x+3-15
-20=x-12
-20x-x=x-12-x
-21x=-12
-21x/-21=-12/-21
x=4/7
 
        
             
        
        
        
It will take 3 hours 6 minutes 40 seconds for the cars to be 420 miles apart.
Step-by-step explanation:
Step 1; The cars are traveling in the opposite direction at different speeds. One is going north at a speed of 75 mph while the other is going 60 mph. So for every hour, the cars are traveling they increase the distance between in between themselves by 75 miles + 60 miles = 135 miles. 
Step 2; Assume that in x hours the distance between them is 420 miles. To calculate x we divide the distance to be traveled by the distance being traveled every hour.
x = distance to be covered / distance being traveled every hour
 = 420 / 135 = 3.11 hours
We multiply the 0.11 hours with 60 to convert it into minutes. 0.11 × 60 = 6.66 minutes and if we do the same for seconds, 0.66 minutes × 60 = 40 seconds.
 
 
        
             
        
        
        
Word form:
six hundred thousand
        
                    
             
        
        
        
One weights 30lbs the other weights 54lbs