Greetings.

Rusting requires oxygen and water to react with iron. Water is plentiful in the set-up, and oxygen comes from the air that is trapped in the inverted test-tube.

Oxygen takes up approximately 21% of air. That comes up to roughly a fifth. Thus, 4 cubic cm of oxygen is about a fifth of the volume of the air initially. That reasons that the original volume of air is 20 cubic cm.

Hope this helps. Regards