The half-life of a radioactive isotope is 10 days. If 3.2 kg are present now, how much was present 30 days previous?

Answer 1

30 days is 3 half-lives. So, there is 1/2^3 = 1/8 as much now as 30 days ago.
That means you had 3.2*8 = ___ kg then.

A(1/2)^(30/10) = 3.2

Answer 2

I don't really get it but the only thing that I think might help is:
10/3.2=a
a*30= answer

Answer 3

oh- thx

Answer 4

That is not one of the answer choices. The choices are 12.8, 25.6, 6.4, 51.2