Find the average weight of four containers, if it is known that the weight of the first container is 100 kg and the total of the second, third and fourth containers' weight is defined by $f\left(x\right)=x^2-\frac{3}{4}{x}^2$ where $x=100$.

Given that the weight of the first container is $100 kg$.

The total of the second, third, and fourth containers weight defined by $f\left(x\right)=x^2-\frac{3}{4}{x}^2$, where $x=100$.

Suppose that the average of four containers is $A$

$\begin{array}{rcl}A& =& \frac{100+{100}^2-{\displaystyle \frac{3}{4}}\left({100}^2\right)}{4}\\ & =& \frac{100+2500}{4}\\ & =& \frac{2600}{4}\\ & =& \boxed{\mathbf{650}\mathbf{ }\mathit{k}\mathit{g}}\end{array}$