If the half-life of uranium 235 is 700 million years and have 64 atoms of it how many 700-million-year units of time will pass before 1 atom of uranium 235 is left?

It would take 1.4 billion years for half of the 64 atoms of uranium-235 to decay, leaving 32 atoms. Then another 700 million years would pass for half of the remaining 32 atoms to decay, leaving 16 atoms. This process continues until only 1 atom is left, which occurs after 5 half-lives or 3.5 billion years.