$X$ can be between 4 and 10
For every number $X$ is below 10, reduce $Y$ by 5%
For example, if $X=7$, then $Y'=0.85Y$
(100 - (5 * (10 - X))) * Y / 100
Can I reduce that expression any further? I want to make it 'cleaner' when the next developer has to look at it.
Here is the first step for reducing the expression above algebraically, using the distributive law of multiplication:
$(100 - (5 \cdot (10 - X))) \cdot Y/100 = (100 - (50 - 5X)) \cdot Y/100$
Proceed further with the other basic rules of Algebra, and you should arrive at Gerry's answer.