Question

Radioactive 24Na with a half-life 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

Answer 1

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.