Hey there!
A half-life means after a certain amount of time, half of that substance will be gone/changed after that time.
There are two half lives in 20 years because 20 ÷ 10 = 2.
So, we divide the 100g sample in half 2 times.
100 ÷ 2 = 50
50 ÷ 2 = 25
There will be 25g of the radioactive sample remaining after two half lives.
Hope this helps!
