If 10 tractors are needed to plow a field in 4 hours, how many tractors are needed to hours, how many tractors are needed to plow the field in 5 hours?

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?