How much water should be added to a 36ml of 20% alchohol solution to reduce the concentration to 18%?

N - ml of alcohol

n1 (alcohol in 20%) = n1 (alcohol in 18%)

0.20*36 ml = 0.18 * x ml

x = 0.20*36/0.18 = 20*36/18 = 40 ml of 18% solution

We had 36 ml, we will get 40 ml,

so we need to add

40 ml - 36 ml = 4 ml of water.