@p00057079 the examples of cracking a cryptographic protocol or finding the most efficient form of compression are examples of computationally hard problems. They require a brute force search over a potentially enormous list of possible answers. They are theoretically "solvable" but could potentially take a very long time.
There is a second class of problems which are unsolvable. The example of determining whether a program will eventually stop (or "halt") is an example of one such problem. It has been proven that there is no fool-proof method for determining whether a given program will eventually halt. In other words, the problem "will this program halt" is unsolvable.
Here's a video on that very problem https://www.youtube.com/watch?v=macM_MtS_w4 that you may enjoy.
To your question of where that specific topic is covered I agree this lesson is no longer clearly addressing that EK. I've noted the issue so that we can make adjustments accordingly. Thanks for taking the time to point out the issue and let me know if we can be of more help with your questions.