Zero's two jobs

A piggy bank held ₹0 on Monday. By Friday it held ₹40.

The rule

Zero does two jobs.

• It can mean none. ₹0 means no money at all.
• It can be a placeholder. It holds a place, so other digits keep their right places.

Example

40 is 4 tens + 0 ones. The 0 holds the ones place. Remove the 0, and the 4 moves into the ones place. Now the 4 means 4 ones. The number is 4, not 40.

305 is 3 hundreds + 0 tens + 5 ones. Remove the 0, and you get 35. Now the 3 is in the tens place. It means 3 tens, not 3 hundreds.