[SOLVED] Half Life

[Natalie]

I can't get the answer to this question.
If Hydrogen 3 ( 3H )
( 1 )
has a half life of 12.5a, how many years have passed when only 1/64 of its original mass remains?

[russ_watters]

12.5a?

Half life is a pretty simple thing, you can count this one on your fingers. 1 half life is 1/2, 2 half lives is 1/4, 3 is 1/8, 4 is 1/16, 5 is 1/32, and 6 is 1/64.

But I don't know what "12.5a" means.