Hans is a film director, and he is attempting to shoot a complicated scene for a movie set in the American West. In the scene, a bank robber tries to make a daring escape atop a moving train. Alongside the train, the movie's hero, a lawman, rides on horseback. Hans plans the scene so that the train will move at a constant speed. The lawman on horseback, while moving in the same direction as the train, will cover the same amount of distance in the same amount of time as the train. The bank robber will move along the train's roof at a speed of 4 meters per second. He will also move in the same direction as the train so that their speeds will combine.

Hans wants to organize the scene so that a camera at rest on the ground will shoot the train as it moves past. He wants the robber, from the camera's point of reference, to be moving at a rate of 25 meters per second.

For this scene to work as Hans has imagined it, how fast must the lawman's horse be able to run?