Question 751561
Interesting you ask, how much water.


If that is how the adjustment is to be done, then ignore the 18% punch and just use the 30% punch.

{{{50*30/(50+w)=25}}} where w is how much liters of water to add.  Solve for w.


{{{25(50+w)=1500}}}
{{{25*50+25w=1500}}}
{{{25w=1500-25*50}}}
{{{25w=250}}}
{{{highlight(w=10)}}} liters water


Certainly there are other ways, such as to use some or (maybe) all(?) of the 18% punch along with the 30%.


A good alternative is say, let e be how much of the 18% punch to add to the 50 liter of 30% punch.  Add no extra water.

... in fact, you would need to add ALL of the 20 liters of the 18% plus some water.
...
{{{highlight((50*30+20*18)/(50+20+w)=25)}}}
Solve for w, the liters of water to add to ALL of the available punch solutions.