Finding a value between two numbers, given percentage, which is bigger when the percentage is smaller

Question

I'd like to find a way to calculate a value which is between two numbers (100 - 500), and that gets smaller the more the percentage is bigger. This means that 100% should give you 100, while 1% should give you 500.

How could I achieve this?

Answer 1

You can use linear function $y=ax+b$. Your requirements are: $100=100a+b$ and $500=a+b$. When we solve this system, we find $a$ and $b$ and the function will be: $$y(x)=\frac{49900-400x}{99}$$