Question

A radioactive isotope has a $t$${}_{1/2}$ of $10$ days. If today there is $125g$ of it left then its weight $40$ days earlier is

• A. $600g$
• B. $1000g$
• C. $1250g$
• D. $2000g$

Answer 1

The correct option is D $2000g$

Amount of material 40 days earlier i.e., x

After 4 half -lives, the fraction of x left is $(\frac{1}{2}{)}^4=\frac{1}{16}$

Given, $\frac{x}{16}=125\Rightarrow x=2000g$