Hey there!
A half-life means after a certain amount of time, half of that substance will be gone/changed after that time.
If 50%, or half, of the element remains after 4000 years, that means the half life must be 4000 years.
Hope this helps!
AgNO3+NaCl yields AgCl+NaNo3 (reduction)
...that's the only one I know
ur mom ur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur momur mom