Posted by Colby on .
Radioactive 24Na with a halflife of 14.8 hours is injected into an animal in a tracer experiment. How many hours will it take for the radioactivity to fall to 12.5% of its original intensity?
I am confused on what number to start with

Chem 
DrBob222,
You have the half life, Use that to calculate k.
k = 0.693/t_{1/2}
Then
ln(No/N) = kt
Use 100 for No
Use 12.5 for N
Substitute k from above.
Solve for t in hours if you use t_{1/2} in hours.