Question

A radioactive isotope has a half life of $10$ days. If today $125$ mg is left over, what was its original weight $40$ days earlier ?

• A. 2 g
• B. 600 mg
• C. 1 g
• D. 1.5 g

Answer 1

Answer: (A)

2 g

Answer 2

$N=N_{0}\left(\frac{1}{n}\right)^{n}, n =\frac{40}{10}=4$
$\frac{125}{1000} =N\left(\frac{1}{2}\right)^{4}$
$N_{0} =\frac{125 \times 2^{4}}{1000}$
$=2 g$