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?

The correct option is A 2 g
Original weight $=$ left weight $\times 2^n$

where,

$n$ is number of half time $=$ elapsed time/half life time

$=40/10$

$=4$

So, original weight $=125\times 2^{4}mg$

$=125\times 16mg$

$=2000mg=2g$