algabre
posted by lee .
the speed of the current in a river is 6 mph a ferry operator who works that part of the river is looking to buy a new boat for his business everyday his route takes him 22.5 miles against the current and back to his deck and he needs this trip in a total of 9 hours he has a boat in mind but he can only test it on a lake where there in no current how fast must the boat go on the lake in order for it to serve the ferry operators needs
can someone please explain?

(22.5/r6)+(22.5/r+6)=9
Multiply the equation by r^236.
Resulting equation: 9r^245r324=0
Simplify: r^25r36=0
Factor: (r9)(r+4)
Answer: rate of ferry= 9mph