Good afternoon!
We calculate the volume of the container in cm³. To do that, we must put the units in cm:
30 cm → 30 cm
50 mm → 5 cm
0.2 m → 20 cm
The volume is:
V = 30 . 5 . 20
V = 3000 cm³
Now, we calculate the mas with the formula:
m = dV
m = 2.5 · 3000
m = 7500 g
Dividing by 1000, we have the mass in kg:
m = 7.5 kg