A radioactive isotope has a half-life of 4 hours. Initially, a sample contains 160 g160\text{ g}160 g of this isotope. What mass of the isotope is left after 12 hours?
The graph below shows the radiation activity emitted from samples of three different isotopes, X, Y, and Z, over a period of 10 days.

Which isotope, X, Y, or Z, has the longest half-life?
Explain why isotope X is initially more hazardous than isotope Y, but becomes less hazardous than isotope Y after 2 days.