Half-life in physics: Plz Help ! :(

C.  40 s

Explanation:

The original 30 atoms is reduced to 15 atoms at 40 s

those 15 atoms are reduce to 7.5 ( call it 8) atoms at 80 s

those 8 are reduced to 4 at 120s

those 4 are reduced to 2 at 160 s

those 2 are reduced to 1 at 200 s

+15-35=-20

going in opposite directions