If the half-life of a radioactive material is 8 years, how many years will it take for three fourths of the original amount of material to decay?

The comparison of the number of atoms in a copper coin the size of a penny with the number of people on earth is made to illustrate which of the following? a. that atoms are indivisible b. that atoms are very small c. that atoms are very large d. that in a copper penny, there is one atom for every person on earth