What is the fraction of the original activity left at the end of the 10 half lives

Question

What is the fraction of the original activity left at the end of the 10 half lives

Answer 1

1/(2^10) , slighly less than 1/1000th.

Answer 2

The general rule that I like to use is that after n half-lives, (1/2)^n of the initial concentration remains.

So, for your case:

n=10

(1/2)^10=1/1024