Rita rows 12 km upstream and 12 km downstream in 3 hours. The speed of her boat in still water is 9 km/hr. Find the speed of the stream.
Suppose the speed of the stream is x km per hour.
When she goes upstream, her speed is x-9 km/hr; when she goes downstream, her speed is x+9 km/hr.
She rows 12 km upstream and takes ...
This provides an example of using quadratic equations to find speed.