How many memory hacks are mentioned in the article?

A series of surveys have shown most students fail to use proven methods of learning effectively, instead wasting their time on ineffective methods.
   Fortunately, a new paper, published in one of the top psychology journals, has examined the biggest misconceptions, with a list of the most popular study strategies, the potential pitfalls, and the ways that they can use them more effectively.
   Strategy 1: Rereading
   Learning new vocabulary? The most common strategy is to read the words and their meanings until they stick. Unfortunately, psychologists believe that it is too passive, meaning that most of the information fails to leave an impression.
   Memory hack: Space your reading
   You can make better use of those sessions, however, by ensuring that you return to the material at regular intervals.
   Strategy 2: Underlining and highlighting
   Like rereading, this study technique is nearly ubiquitous. But although it can be more effective than passive rereading, underlining and highlighting often fails to work, with most students mindlessly marking up almost every paragraph without much discernment.
   Memory hack: Pause to think
   Instead, the scientists suggest that you read the text once through cold, and then mark up the relevant passages on the second pass.
   Strategy 3: Note-taking
   Visit any lecture theatre or library and you will find students judiciously copying the most important facts into their notebooks. Like underlining and highlighting, the problems come when you fail to be judicious about the material you are including.
   Memory hack: Be concise
   Experiments have shown that the fewer words students use to express an idea in their notes, the more likely they are to remember it afterwards. This is probably because creating summaries and paraphrasing force you to think deeply about the nub of the idea you are trying to express — and that additional effort cements it in your memory. These findings may also explain why it is better to take notes with a pen and paper, rather than using a laptop. Writing by hand is slower than typing and forces you to be more concise in what you note down.