If we start with 120 atoms of a radioactive substance, how many will remain after three half-lives

can you explain how to do this?

Hey there!

A half-life means after a certain amount of time, half of that substance will gave decayed after that time.

So, we divide the 120 atom sample in half 3 times.

120 ÷ 2 = 60

60 ÷ 2 = 30

30 ÷ 2 = 15

There will be 15 atoms of the radioactive sample remaining after three half lives.

Hope this helps!