On the first day, the soup reduces down to 80/2=<<80/2=40>>40 kg.
On the second day, it reduces down to 40/2=<<40/2=20>>20 kg.
On the third day, it is reduced to 20/2=<<20/2=10>>10 kg.
And on the fourth day, it is finally down to 10/2=<<10/2=5>>5 kg.
#### 5