Answer:
0.4 kg/m^3
Step-by-step explanation:
v (volume) = 20m x 10m x 5m = 1000 m^3
w (weight) = 400 kg
d (density) = w / v
d = 400 / 1000 = 0.4 kg per cubic meter (kg/m^3)
I apologize if this is a little complex, however all of the math is there, if you're interested then look it over. Otherwise I've provided the answer in the units used for the dimensions and weight provided.