It takes 10 minutes for a canoeist to paddle perpendicular to the current
across a river of uniform width. (Of course, while crossing the river the
canoeist drifts downstream.) The canoeist then paddles upstream for 50
minutes, recrosses the river and paddles downstream for 20 minutes,
returning to the starting point. What is the ratio of the canoeist’s
speed in still water to the speed of the river current?