Question 10567
Let his hourly cost = c

Then for 3 hours he will charge 3c

So his total cost = X+3c   where X is his fixed fee

X+3c = 150

X+ 6c = 180

Subtracting the first eqn from the second we get  3c = 30 or c= 10

Substituting in any equation we get X = 120

So for a 9 hour job he will charge X + 9c = 120  +9x10 = 120+90 = $210