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.