Question

A sample initially contain 80.0g of an isotope after 24.0hrs only 5mg of the sample was left what is the half life of the isotope

Answer 1

5/80 = 1/16, so 4 half-lives = 24 hrs ...

Answer 2

Note: oobleck's solution is quite right, esoteric and intuitive. For those who prefer to use equations and not intuition here is the method.
ln (No/N) = kt where No is starting, N is what's left, k is a constant and t is the time. So
ln(80/5) = k(24 hrs)
ln(16)/24 = k
2.773/24 = k and
k = 0.1155, then
k = 0.693/half life
half life = 0.693/0.1155 = 6