> Alternative Method -
Ratio = $\left( 1 - \frac{a}{x + y} \right)^n$
a is amount of water added.
x is quantity of wine.
y is quantity of water.
You can simply say x + y total solution.
n is number of time process repeated.
= $\left( 1 - \frac{4}{30 + 10} \right)^2$
= $\left( 1 - \frac{4}{40} \right)^2$
On solving you get 81:100