Question

A radioisotope has a half life of $10$ days. If today there is $125$ $g$ of it left, what was its mass $40$ days earlier?

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

Answer 1

The correct option is D $2000$ $g$

Initial sample be $xg$.

40 days = 4 half lives

Sample after 4 half lives = $\frac{x}{16}$

$⟹\frac{x}{16}=125g$

$⟹x=2000g$