The percent of remaining parent isotope in a radioactive decay process is 40 percent. how many half-lives have elapsed since the material was 100 percent parent atoms?

Between 1 and 2.

Explanation:

Half life is the amount of time taken by a radioactive material to decay to half of its original value.

       ............(1)

where,

a = amount of reactant left after n-half lives  = 40

= Initial amount of the reactant  = 100

n = number of half lives

Putting in the values we get:

taking log on both sides

Thus half-lives that have elapsed is between 1 and 2