The half-life of a radioactive isotope is $3 \ hours$. If the initial mass of the isotope were $256 \ g$,the mass of it remaining undecayed after $18 \ hours$ would be ....... $g$. (in $.0$)

The half-life of a radioactive substance is $120$ days. After $480$ days,the amount of $4 \ g$ of the substance remaining will be: (in $g$)

The half-life of a radioisotope is $20 \ hours$. After $60 \ hours$,what fraction of the initial amount will remain?

If the half-life of a certain radioactive nucleus is $1000 \ s,$ the disintegration constant is

What is the percentage of a radioactive element decayed after $20 \ s$ if its half-life is $4 \ s$?

$A$ radioactive element gets spilled over the floor of a room. Its half-life period is $30$ days. If the initial activity is ten times the permissible value,after how many days will it be safe to enter the room?