Final answer:
To leave your film in the tank in the darkroom only half as long as you need, you should decrease the exposure time.
Step-by-step explanation:
To leave your film in the tank in the darkroom only half as long as you need, you should decrease the exposure time. Decreasing the exposure time means that the film will be exposed to less light, resulting in a lighter and less developed image on the film. By reducing the exposure time, you can control the level of darkness in the developed film.