# Question #0ff3e

$\text{2 days}$
In your case, you start with $\text{10 g}$ of a radioactive substance. Two days after first interacting with the sample, you find that its mass decreased to $\text{5 g}$.
This is, of course, equivalent to saying that $\text{5 g}$ of the substance decayed. Since it took $2$ days for half of the sample to undergo radioactive decay, you can say that this radioactive isotope has a half-life of $2$ days.