Final answer:
The expression 20-x decreases as x increases because the increasing variable amount is subtracted from a constant, resulting in a smaller overall value.
Step-by-step explanation:
The value of the expression 20-x decreases as x increases because x is being subtracted from 20. In mathematics, if you have a constant number and subtract a variable amount (x) that is increasing, the overall value of the expression will go down. Consider the function f(x) = 20 - x; as x increases within the interval from 0 to 20, the value of the function f(x) decreases. This is easily seen on a graph where the function will have a downward slope from left to right.