Arock is suspended from a scale reads 20.0 n. a beaker of water is raised up so the rock is totally submerged in the water. the scale now reads 12.5 n. what is the density of the rock?

The rock's density is equivalent to 2667 kg/m3.

mass of rock = F / g (gravitational constant)
                      = 20/ 9.8
                      = 2.039 kg
While submerged in water, the rock has displaced (20.0 N - 12.5 N =) 7.5 N worth of water, This convert to the mass: 0.7645 kg.
 
The density of water is 1000 kg/m3, so the volume of water displaced is (0.7645 kg) / (1000 kg/m3) = 7.645 x 10^-4 m3.

density of rock = mass / volume
                        =  0.7645 kg/7.645 x 10^-4 m3. 
                        = 2667 kg/m3.

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