flash maths - scaling a number

Hi guys,

I need help with a math problem

I need a math formula to scale a number from between 400 and 10000
to a number between 800 and 8000

In other words 400 would return a value of 800
10000 would return a value of 8000
and any number between 400 and 10000
would return a corresponding answer between
800 and 8000 the same percentage along the span of numbers.

For example the number half way between 400 and 10000
would return the number half way between 800 and 8000