UX/Content issue in lessons with sub variables

# Description
When doing the exercises where sub-variables need to be accessed, for example, in lesson 20, when you need to write the for loop and access `sizes.x and sizes.y` in `rectangle_sizes`, there is nowhere in the instructions that is explicitly defined.

I have been programming for many years on and off, though my main job is UX Research for 15+ yrs. I was able to write the code correctly, with proper syntax. In this case, I just never knew what you had named the variables I needed to access, since we can't see all of the code. This feels like a basic recognition over recall usability problem, unless you're leaving that out for a specific reason. If you are, at the very least, there needs to be some sort of signifier there that will guide the user to how they can resolve the problem on their own. I have found myself being forced to open up the solution to see what the variables are named, and that takes a lot away from the experience. I would much rather have a way to figure it out myself or decipher your naming convention instead of looking at the solution, much more rewarding.