Question

A radioactive isotope has a half-life of $10$ days. If today there is $125$ gms of it is left, its weight $30$ days earlier was

• A. $250gms$
• B. $1000gms$
• C. $500gms$
• D. $2000gms$

Answer 1

The correct option is B $1000gms$

Let amount left 30 days earlier $=x$
Half life $=10days$
Fraction of amount left after 3 half lives i.e., 30 days
$(\frac{1}{2}{)}^3=\frac{1}{8}$

Given, $\frac{1}{8}\times x=125gms\Rightarrow x=1000gms$