Answer:
20 gallons were used from the tank with the octane rating of 80.
Step-by-step explanation:
Given : One tank of gasoline has an octane rating of 140 and another tank of gasoline has an octane rating of 80. To obtain a mixture of 60 gallons with an octane rating of 120.
To find : How many gallons should be used from the tank with the octane rating of 80?
Solution : Let x be the volume of gas whose octane rating is 140 and 
y be the volume of gas whose octane rating is 80.
Then, we form the equation as: 
(1) 
 
or   .........[3]
 .........[3]
(2) 
Now multiply equation (2) by 4  
 ...........[4]
 ...........[4]
Subtract (4) from (3)  
 
 



Put x in equation [4] we get,





Therefore, x=40 and y=20
So, 20 gallons were used from the tank with the octane rating of 80.
 
        
             
        
        
        
In About 13 days the water in the tank will hit 6000 gallons