How much time would it take for 12000 atoms of nitrogen-13 (half-life is 7.13 s) to decay to 750 atoms?

Question

a)35.65 s

b)28.52 s

c)42.89 s

d)21.39 s

Answer 1

12000 (1/2)^(t/7.13) = 750
now solve for t

or, note that 750/12000 = 1/16, or 4 half-lives