A nail is a tool for connection and is a industrial product.
I use the nail to cut the meat.
20 times.
The use of the tool is inappropriate.
It adds up(accumulates).

A screw is a tool for connection and an industrial product.
I use a nail to connect the divided meat.
10 times
The use of the tool is appropriate.
I take it away. (I consume.)

The remaining is 10

The ground is formed under the judgment that repetition of the action of accumulating individuals of appropriate division is ethical.

I had an exhibition at a re-development site.
The condition demanded by the dwellers of the to-be-destroyed homes was 18 pyong apartments.
18.
But the value of their existence assessed by society was less than that.