Volume
Formula
V = l*w*h
Solution
l=50 cm
w= 40 cm
h = 0.20 cm
V = 50*40*0.2
V = 400 cm^3
Density
D = m/V
D = ???
m = 60 grams
V = 400 cm^3
D = 60 grams / 400 cm^3 = 0.15 grams / cm^3
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Kg/m^3
You should do the kg / m^3 by using dimensional analysis.
0.15 grams / cm^3 [ 1 kg / 1000 grams] * [100 cm * 100 cm * 100 cm / 1 m^3]
0.15 grams / cm^3 [ 1 kg / 1000 grams] * [1,000,000 cm^3 / 1 m^3]
0.15 grams /cm^3 [1,000,000 cm^3 / 1000 grams ] [ kg/m^3}
0.15 * 1000 kg/m^3 = 150 kg / m^3
The point of this is that everything else cancels out.
So you have two answers
D = 0.150 grams / cm^3
D = 150 kg /m^3
