Hey there!
A half-life means after a certain amount of time, half of that substance will be gone/changed after that time.
32 days is two half lives because 32 ÷ 16 = 2.
So, we divide the 64g sample in half 2 times.
64 ÷ 2 = 32
32 ÷ 2 = 16
There will be 16g of the radioactive material remaining after 32 days.
Hope this helps!