Answer:
c≠20 or if you are writing it in interval notation c can be negative infinity to 20 not including 20 and 20, though not including 20, to infinity.
Explanation:
Forst let's get them into the form of y=mx+b
y=10-2.5x and y=.5c-2.5x
Now, if we graph any two equations, the only way to ensure there is never an answer is to make sure they never touch. This means they have to be parallel, or in other words have the same slope but different y intercept.
The two equations already have the same slope, -2.5, so that means they just can't have the same y intercept. the first one has a y intercept of 10, so that means the other one cannot have that. The y intercept for the second one is .5c, so in otherwords .5c can't equal 10, or c≠20.