Hey there!
A half-life means after a certain amount of time, half of that substance will have decayed after that time.
There are 3 half-lives in 72,000 years because 72,000 ÷ 24,000 = 3.
So, we divide the 100g sample in half 3 times.
100 ÷ 2 = 50
50 ÷ 2 = 25
25 ÷ 2 = 12.5
There will be 12.5g of the radioactive sample remaining after 72,000 years.
Hope this helps!
