Hey there!
A half-life means after a certain amount of time, half of that substance will be gone/changed after that time.
So, we divide the 16g sample in half 3 times.
16 ÷ 2 = 8
8 ÷ 2 = 4
4 ÷ 2 = 2
There will be 2g of the radioactive sample remaining after three half lives.
Hope this helps!