$A$ radioactive substance has a half-life $(t_{1/2})$ of $60 \ minutes$. After $3 \ hours$,what percentage of the radioactive substance will remain?

$1.0 \ g$ of radioactive sodium decays to $0.25 \ g$ in $16 \ hours$. Calculate the time required for $48 \ g$ of the same radioactive sodium to decay to $3.0 \ g$. (in $hours$)

$1.0 \, g$ of a radioactive isotope was found to reduce to $125 \, mg$ after $24 \, hours$. The half-life of the isotope is $....... \, hours$.

$_{11}Na^{24}$ has a half-life of $15 \ h$. On heating,its half-life will:

$2 \text{ g}$ of a radioactive sample having a half-life of $15$ days was synthesized on $1^{st}$ Jan $2009$. The amount of the sample left behind on $1^{st}$ March $2009$ (including both the days) is: (in $\text{ g}$)

The amount of $_{53}I^{128}$ $(t_{1/2} = 25 \ minutes)$ left after $50 \ minutes$ will be