Final answer:
To rearrange dartboards on the screen using the 'arrange by now' layout, use the Object > Arrange > Layout option.
Step-by-step explanation:
To rearrange all dartboards on the screen using the 'arrange by now' (left to right) layout and set the spacing to 100px, you can use the Object > Arrange > Layout option. This allows you to arrange objects on the artboard in a specific order and distance. You can then select the 'left to right' option and set the spacing to 100px.
Remember to make sure that the artwork stays on the artboard. If the dartboards go off the artboard when you arrange them, you can resize the artboard or move the artwork closer to the center so that it fits within the boundaries.