Because it is proportional to the respected numbers square. 4^2=16 and because it is proportional 80/16=5. We have to square 8 to get 64 and then multiply that value by 5 to get 320.
\(d\; \alpha\; t^2\\ d=kt^2\\ find\;k\\ 80=k*4^2\\ k=5\\ d=5t^2\\ after\;12\;sec\\ d=5*12^2\\ d=720m\\ 720-80=640 \)
So in the next 8 sec the ball will drop 640m