The speed of the boat is 21 km/h, and the speed of the stream is 5 km/h.
Let's denote the speed of the boat as b and the speed of the stream as s. When the boat is traveling downstream, it benefits from the added speed of the stream, so the effective speed is b+s. Conversely, when the boat is moving upstream, it faces the opposing speed of the stream, resulting in an effective speed of b−s.
The formula relating speed, distance, and time is Speed=Distance/Time. Using this formula, we can set up two equations based on the given information:
For downstream travel: b+s= 78km/3h
For upstream travel: b−s= 64km/4h
Solving these simultaneous equations, we find b=21km/h and s=5km/h. Therefore, the speed of the boat is 21 km/h, and the speed of the stream is 5 km/h.