Although I really like the french idea, I want to offer another, rule-bending, idea:

Put in a decimal point (or a comma, depending where you live) and make it 4.20109

Now the number is definetly below 100 without breaking the rules:

• Putting in a single pixel is as easy as it gets (and we don't need to learn another language!)
• Not a mathematical funciton
• No hiding or erasing of numbers
• Not a mathematical operator (although a mathematical symbol)

Although I really like Bass' idea, I want to offer another, rule-bending, idea:

Put in a decimal point (or a comma, depending where you live) and make it 4.20109

Now the number is definitely below 100 without breaking the rules:

- Putting in a single pixel is as easy as it gets (and we don't need to learn another language!)
- Not a mathematical function
- No hiding or erasing of numbers
- Not a mathematical operator (although a mathematical symbol)