If 9 five-rupee coins push up 10 mL, then 18 coins push up:

1. 9 coins push up 10 mL of water. 2. 18 coins is twice 9, so they push up twice as much. 3. That is 20 mL. _Source: syllabus Mathematics, 'Math-Magic', Ch 14 'How Big? How Heavy?'_