concentration = (solute/solution) *100

given concentration = 15%

let amount of water be x

solute = 15ml

solution = 15ml + xml

(15/15+x)*100 = 15

15/15+x = 15/100

15/15+x = 3/20

15 = (3/20)(15+x)

15 = (3/20)15 + (3/20)x

15 = 9/4 + 3x/20

15 - 9/4 = 3x/20

51/4 = 3x/20

51*20/4 = 3x

51*5 = 3x

51*5/3 = x

x = 85ml