When solving mixture questions, I find it useful to sketch the solutions with the ingredients SEPARATED.

Since we're asked to find a PERCENTAGE, we can assign a "nice" value to the original volume.
So, let's say we start with 100 liters

As you can see, 50 liters is alcohol and 50 liters is water


Now let's remove x liters of the mixture from the container.
Half of removed x liters will be alcohol, half of the removed x liters will be water
In other words, we're removing 0.5x liters of alcohol, and 0.5x liters of water.
So, the resulting mixture looks like this:



We're going to replace the x liters of missing solution with x liters of 25% alcohol solution


When we add the volumes of alcohol and water, we get a final mixture that looks like this:


Finally, we want the final mixture to be 30% alcohol.
In other words, we want: (volume of alcohol)/(total volume of mixture) = 30/100 (aka 30%)
We get: (50-0.25x)/100 = 30/100
Cross multiple: (50-0.25x)(100) = (100)(30)
Simplify: 5000 - 25x = 3000
Solve to get: x = 80

So, 80 liters were originally removed

Answer: E