Hey there!
A half-life means after a certain amount of time, half of that substance will be gone/changed after that time.
4 hours is 4 half lives because 4 ÷ 1 = 4.
So, the 200g sample will be divided in half 4 times.
200 ÷ 2 = 100
100 ÷ 2 = 50
50 ÷ 2 = 25
25 ÷ 2 = 12.5
There will he 12.5g of neptunium-240 left after 4 hours.
Hope this helps!
