A riverboat travels at an average of 9 miles per hour in still water. The riverboat travels 90 miles up the miami river and 90 miles down the same river in a total of 19 hours. What was the speed of the current of the river?

Ben frank is an electrician who installs lighting fixtures. he charges (in dollars) his clients 15h+40f15h+40f where hh is the number of hours worked and ff is the number of fixtures installed. how much does ben frank charge when he works 77 hours and installs 99 fixtures?