Well...if he earns $75 an hour....and he worked for 20 hours...that's
75 * 20 which = 1500
Now it says he also earns a flat fee....since the question states he billed the client 1800...and he only earned 1500 of it...that must mean that his flat fee would be
1800 - 1500 = 300
So his flat fee is 300...and his variable charge...is 75x (75 dollars per hour)
in an equation...this would look like
C(x) = 75x + 300