If you have 24 grams of an unstable atom with a half life of 5,000 years, how many grams would be left after 15,000 years?

Answer 1

k = 0.693/t_1/2

Then substitute k into the equation below.
ln(No/N) = kt
No = 24
N = solve for this
k from top equation
t = 15,000
Post your work if you get stuck.
Of course the much easier way is to say you go through three half lives so you are left with 1/8 of 24 = 3 grams.