So I've discovered there is no option to effectively enter fractions. I've a few choice words to say about that. However...
I've managed to draw a simple shape, the first part in a more complex tool, of two rectangles joined. Where the line overlaps I want to trim it to make one L shape. Snap will not work with trim. Trim will not select either entity. What the hell is going on? I'm about to go back to paper and a slide rule!!!!
When you draw a rectangle, it is created as polyline to keep a relation between the entities.
If you want to trim the rectangle, you have to use Modify -> Explode first, to separate the lines again, before trimming.