Percentage Removed Equal to Remaining Average

Each of 4 fluid containers containing $ 100 \, \text{mL}, 200 \, \text{mL}, 300 \, \text{mL} $, and $ 400 \, \text{mL} $ is reduced by the same percentage $ x $ of its volume. After doing this, the volume of the removed fluid is equal to the average of the volumes of the remaining fluids in the four containers. What is the value of $ x $?