Answer:
14,146 years
Explanation:
The annual multiplier is (1 -0.0049%) = 0.999951. We want to find the number of times this needs to be multiplied to make the product 1/2.
0.5 = 0.999951^t
log(0.5) = t·log(0.999951)
log(0.5)/log(0.999951) = t ≈ 14,145.514
The half-life is about 14,146 years.