Truthfully, efficiency isn't all it's chalked up to be. While it can sometimes be helpful to be fairly quick at solving problems (e.g. on a test), it's far more important to get the right answer, even if it takes you a bit longer. The only tiny thing I'd caution about your work is that instead of plugging the answer you want to check back into one of the equations, you'll want to check if your answer makes sense in the context of the problem text. It's possible your answer checks with the second

equation, but it's the wrong equation for the problem. In your case, your answer is that the tanks each started with 4200 liters in them. If 3000 liters is pumped from one tank into the other, the tanks have 1200 and 7200 liters, respectively. 7200 liters is six times as much as 1200 liters, so the answer checks out.

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