Radioactive material obeys 1st order decay kinetics,
For 1st order reaction, we have
k =

where, k = rate constant of reaction
Given: Initial conc. 100, Final conc. = 6.25, t = 18.9 hours
∴ k =

= 0.1467 hours^(-1)
Now, for 1st order reactions: half life =

= 4.723 hours.
The last row going across
Answer:
c) +2 to 0
Explanation:
SO4 has a charge of -2, so the Cu attached to that has to be a +2 since the polyatomic molecule has no overall charge
Cu(s) is a solid metal and they have no charge, therefore it is zero
Copper undergoes Oxidation (gain of electrons)