How can scientists tell when the reversals of the magnetic field occurred?

There are 6.022 x 10^23 atoms of hg in 1 mole of hg. the number of atoms in 4.5 moles of hg can be found by multiplying 4.5 by 6.022 x 10^23 a. 2.7 x 10^24 b. 27 x 10^23 c. 2.71 x10^24 d. 27.099 x 10^23

Harvey mixes two liquids. which observation of the new mixture most likely indicates a precipitate is forming?

V1t2 = v2t1 is an expression of who’s law