Hey there!
A half-life means after a certain amount of time, half of that substance will be gone/changed after that time.
60 hours is 4 half lives because 60 ÷ 15 = 4.
So, the 18g sample will be divided in half 4 times.
18 ÷ 2 = 9
9 ÷ 2 = 4.5
4.5 ÷ 2 = 2.25
2.25 ÷ 2 = 1.125
There will he 1.125g of sodium-24 left after 60 hours.
Hope this helps!
