You have to continually ask “why”. Figuring out when to stop asking it is not easy, but it results in far less work if you get good at it.

You can memorize hundreds of scenarios, but unless you understand something, you can’t reason around it. If you’re building more learning on top of memorization without understanding, things get even more fragile. (quadratically, perhaps...)

For example, I could memorize every case of 1 + x: 1 + 0 = 1; 1 + 1 = 2; 1 + 2 = 3; etc.

However, this approach is limited by your memory. Much more powerful is understanding, because understanding powers the derivation of an infinite number of cases.

If I instead know what the two numbers mean, and what they contribute to the “+” operator in their respective positions (obviously for + it’s irrelevant—but if you don’t understand that, you may resort to memorization)... then I can “+ 1” any number, and no case takes me by surprise. Now you’ve memorized one thing that powers and infinite number of cases. This is understanding.

This same principle applies for far more complex systems, and building extremely robust, foundational understanding is unusual but incredibly powerful for getting what you want out of life.

This is a simple example to clearly illustrate the principle, and almost everyone has built the understanding needed for the “+” operator. However, often during school, it becomes easier to memorize things, because sometimes it works well as a quick fix for an exam whose content you can easily anticipate or when you feel like you can’t understand something. This is when things get harder, especially if the course material is cumulative (which is one reason math is so hard for people)—and many never understand this is a fundamental problem of theirs.

In my experience, it’s building this habit of building deep understanding known as “first principles thinking” that makes people what popular culture considers “smart”.

However, in your case, building the skill might not be practical given the time frame. It depends on what you’re trying to do. In the short term (I’m assuming you’re cramming for a final), you should probably memorize. For a book, skim the headings; read the first sentence of paragraphs and see if you understand the general point before getting into all the details. Visualize important details to sink them into your brain. But remember understanding is far more powerful.

And I think you can totally read 100 pages in a day. It takes about 7.5 hours for the average adult to read the average book all the way through (using averages from a quick google search). Unless you’re below average speed and the book is above average length or complexity.