A speedboat moves at a rate of 21 km/hr in still water. How long will it take someone to ride the boat 60 km downstream if the river's current moves at a rate of 9 km/hr?
To find out how long it will take someone to ride the boat 60 km downstream, we need to consider the combined effect of the boat's speed and the river's current.
The boat's speed in still water is given as 21 km/hr. This means that if there were no current, the boat would travel at a constant speed of 21 km/hr.
However, we also need to account for the river's current, which moves at a rate of 9 km/hr. Since the current is moving in the same direction as the boat is traveling downstream, it will help to increase the overall speed of the boat.
To calculate the combined speed of the boat and the current, we add the boat's speed in still water to the speed of the current:
Combined speed = Boat's speed + Current's speed
Combined speed = 21 km/hr + 9 km/hr
Combined speed = 30 km/hr
Now that we know the combined speed, we can use it to determine the time it will take to travel 60 km downstream. The formula to calculate the time is distance divided by speed:
Time = Distance / Speed
Time = 60 km / 30 km/hr
Time = 2 hours
Therefore, it will take someone 2 hours to ride the boat 60 km downstream with a river current of 9 km/hr.