Exposure Triangle?

I am currently the teaching Certificate IV Students how to use their digital cameras more effectively. One task revolves around using a simple calculator to work out a series different exposures. All ‘correct’ but some giving different outcomes based on shutter or aperture choice. I don’t allow the students to change ISO in this task. This idea is often referred to as the exposure triangle. In the days of teaching film the ISO was always locked in a single place. Students then only needed to understand the relationship between image brightness and aperture and shutter.
I was unhappy with the way the students responded to this task.
In an effort  to find another way of explaining or teaching it I opened three books. Those books were:- 'Reframing Photography Theory and Practice’, 'Photography 4.0 A teaching guide for the 21st Century' and 'Digital Photo Assignments, Projects for all levels of Photography Classes'. None of these excellent books draws a string around the 3 principles involved in image brightness and exposure? Is this a deliberate choice? Do the editors and writers of these books not consider the relationship meaningful enough for a section or a paragraph?
A quick google returns an enormous amount of information. It seems I will have my work cut out for me formulating a new way to teach this idea.

[edit 2019-04-03] I found two more books in my library that could offer some interpretation or explanation. These are, Langford's Basic Photography and Horenstien's Black and White Photography, a  Basic Manual. Both are pre-digital books and as a consequence approach the idea from a film angle. That is; that it, exposure is fixed at the ISO level. In good news. I took my students out to practice. The idea may be sinking in now. we shall see?

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