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!