A boat can travel at a speed of 20km/h in still water. The current in a river flows at 5km/h so that downstream the boat flows at 25km/h and upstream it travels at only at 15km/h. The boat has only enough fuel to last 3 hours. The boat leaves its base and travels downstream. Draw a distance-time graph and draw lines to indicate the outward and return journeys. After what time must the boat turn back around so that it can get back to base without running out of fuel?