A certain river has a current of 3 miles per hour. A boat takes twice as long to travel upstream between two points as it does to travel downstream between the same two points. What is the speed of the boat in still water?

Let Boat speed = x mph

current = 3 mph

against current speed = x-3

with current speed = x+3

let y be the distance .

downstream t= y/(x+3)............(1)

upstream 2t = y/(x-3)..............(2)

multiply (1) by (2) ,

2t=2y/(x+3)

Therefore y/(x-3)=2y/(x+3)

1/(x-3)=2/(x+3)

2(x-3)=x+3

2x-6=x+3

2x-x=6+3

x=9 mph speed of b