Solve Half-Life Problem: Find # Atoms Decaying in 12 Days

[VjStylz]

Alright so i have exams in a matter of days and stumbled upon this multiple choice question

7. A radioactive source has 1.6×10^20 atoms of a radioactive isotope, with a half-life of 3 days. How many atoms will decay in 12 days?
A. 1.0×10^19
B. 1.2×10^20
C. 4.0×10^19
D. 1.5×10^20

So my first obvious choice of discourse was to attack the question in terms of proportionality:

3 = 1.6 * 10^20
12 = x
And my answer was : 6.4 * 10^20, Which is not even close to any of the above options. I reall don't know what to do from here. Please Help, Thanks in advance

 

[Orodruin]

3 = 1.6 * 10^20

This statement makes absolutely no sense. Three is not equal to 160000000000000000000.

You are also stating that more atoms have decayed than you had from the beginning, which is clearly not physically reasonable.

If the half life is 3 days, how many atoms do you have left after 3 days? How many of those do you have left after another 3 days?