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"What is the density of a rock (mass 75g) submerged in water which displaced by a volume of 37.5cm^3? SI units"

The density is defined as the measure of mass per unit volume:

ρ = mV

Where: ρ =density, m=mass, V=volume

The mass of the rock is given as 75 g, but we need this in SI units of kilograms (kg):

m = 75/1000 = 0.075 kg
Fortunately, the volume of water that the rock displaces is exactly equal to the volume of the rock (an exciting discovery that Archimedes found, compelling him to yell “Eureka!”). Converting this to cubic meter:
V = 37.5/1000000=3.75 × 10-5 m 
Plugging in all the values, we get our final answer:
ρ = 0.075/3.75 × 10-5 =2000 kg/m3

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