Important concepts: Mass varies directly with Volume
Mass = Constant * Volume
Mass = 100 kg
Volume = 80 L
100 = k * 80
100 / 80 = k
1.25 = k
NEW Mass = 3.2 kg
Mass = Constant * Volume
3.2 = (1.25) * V
3.2 / 1.25 = V
2.56 = V
Your answer is 2.56 Liters! Hope that helps!