13.0mL of ethanol x 0.79 g/mL = 10.27 grams of ethanol.
The ethanol will be dissolved into the solution that's been made up to 100 mL.
(a) 10.27 g / 100mL of solution
(b) 10.27 g/ 100mL = 102.7 g / litre (divide answer (a) by 0.1)
(c) Based on 100 ml of solution, (10.27 g / 100 mls) * 100 = 10.27 % (w/v)