A boat in still water travels at x miles per hour. With the current it travels 10 miles per hour faster. If the entire trip up and back takes 10 hours and half the trip is 5 miles, how fast is the boat in still water?

Now try this:

With the current a swimmer goes 3 miles per hour faster downstream than upstream. If one of the trip is 2 miles and the total time for a round trip is 4 hours, what is the speed of the swimmer in still water?