Hi there! The $75 is a one time fixed price and $52.50 is charge per hour. To find amount of total charge, you multiply the charge per hour by number of hours worked, in this case, 4.5. 52.5 * 4.5 is 236.25. That’s $236.25 for 4.5 hours of work. Now, add the $75 to get the total price, when you do, you get 311.25. There. The total charge for the plumber is $311.25.