How much water should be mixed with 12 mL of alcohol so as to obtain 12% alcohol solution?

Volume of solute = 12 ml

Volume of solution = volume of solute + volume of solvent    

Volume of solvent = volume of solution – volume of solvent

= 100 – 12 = 88 ml

$concentration= \frac {volume\ of\ solute}{volume\ of\ solute + volume\ of \ solvent}×100$  

$12=\frac {12}{12+x}×100$

$x=88$

Hence, 88 ml of water will be added to 12 ml of alcohol in order to prepare a concentration 12%.