Atoms of element a decay to atoms of element b with a half-life of 20,000 years. if there are 10,000 atoms of a to begin with (and 0 atoms of b), how long will it take for there to be 2,500 atoms of a?

         atoms    10000         5000      2500

         Half life      0                1              2

From the above description, we have 2500 atoms left after 2 half lives.

=  2 * 20000 =  40000

That is after 40000 years.

To use anything transparent

It's have to be 12 or idk