If mixture A has 20% ethyl alcohol and rest water, and mixture B has 20 % ethyl alcohol, rest water too. When both are mixed in, the resulting mixture formed is called Delta. Upon mixing these three mixtures, another solution is prepared called Gamma. And it is given that Gamma has 20% ethyl alcohol too.
How to prove that the ratio in which all three mixtures are mixed together is irrelevant as mixture Delta will be having 20% ethyl alcohol too.
The proof is straight forward.
Suppose you take $a$ litters of A, $b$ litters of B and mix them up. Your will have $\frac{a+b}{5}$ litter of ethyl alcohol, which is $20\%$ of total volume of Delta.
Now suppose, we take $a_1$ litters of A, $b_1$ litters of B and $c_1$ litters of Delta. The volume of ethyl alcohol will be $\frac{a_1+b_1+c_1}{5}$, which is $20\%$ of total volume of Gamma.