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 5 half lives in 40 days because 40 ÷ 8 = 5.
So, we divide the 400g sample in half 5 times.
400 ÷ 2 = 200
200 ÷ 2 = 100
100 ÷ 2 = 50
50 ÷ 2 = 25
There will be 25g of iodine-131 remaining after 40 days.
Hope this helps!
