Let X = the ORIGINAL volume of fuel in Tank X
Let Y = the ORIGINAL volume of fuel in Tank Y

Tank X holds 600 gallons more than tank Y
We can write: X = Y + 600


If 100 gallons of fuel were to be pumped from each tank, tank X would then contain 3 times as much fuel as tank Y
So, X - 100 = NEW volume of fuel in Tank X
And Y - 100 = NEW volume of fuel in Tank Y
We can write: X - 100 = 3(Y - 100)
Expand to get: X - 100 = 3Y - 300
Rewrite as: 3Y - X = 200

What is the total number of gallons of fuel in the two full tanks?
In other words, what is the value of X + Y?

We have the following:
X = Y + 600
3Y - X = 200

Take the bottom equation and replace X with Y + 600 to get: 3Y - (Y + 600) = 200
Simplify: 2Y - 600 = 200
So, 2Y = 800
Solve: Y = 400

Since, X = Y + 600, we can replace Y with 400 to get: X = 400 + 600
So, X = 1000

This means X + Y = 1000 + 400 = 1400

Answer: A

Cheers,
Brent
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