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

The correct option is A 2 g

$N=N_0{\left(\frac{1}{2}\right)}^n.n=\frac{40}{10}=4$
$\frac{125}{1000}=N_0{\left(\frac{1}{2}\right)}^4,N_0=\frac{125}{1000}\times 2\times 2\times 2\times 2=2g$