)A rock is put in a graduated cylinder with initially contained 15.0mL of water. The level rose to 17.5 mL and the rock had a mass of 7.50g. What is the density of the rock to the correct number of significant figures?

  3.0 g/mL

Explanation:

The volume is the difference in water levels:

  17.5 mL -15.0 mL = 2.5 mL

The density is the ratio of mass to volume:

  (7.50 g)/(2.5 mL) = 3.0 g/mL

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Taking the difference of "large" numbers reduces the number of significant figures to 2, so that is the number the answer must use. All other numbers here have 3 significant digits.

c-14 is carbon-14, or radiocarbon.   it is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.