A 20kg solution is 15% alcohol. How much water should I add to make an 8% alcohol solution?

17.5 kg of water.

Step-by-step explanation:

The amount of alcohol in the 15% solution = 20*0.15 = 3kg.

If 3 kg is equivalent to 8% solution

then 100% is 3*100 / 8 = 37.5 kg.

So the amount of water to be added to make 8% solution = 37.5 - 20

= 17.5 kg.