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Missing homework template, originally posted in different forum.

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 dont know what to do from here. Please Help, Thanks in advance

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 dont know what to do from here. Please Help, Thanks in advance