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<p>The teacher did examples. Sometimes they would write a problem down and give people a little bit of time to think about it. Giving people a page number in the book to look up would be spectacularly inefficient. </p>
<p>And just having people do a bunch of problems isn’t really appropriate for class. The point of class time (i.e. contact time with the teacher) is that the teacher actually teaches you, and doing a bunch of problems by yourself then seeing the answers afterward doesn’t really take advantage of that. That’s what homework is for.</p>
<p>When the teacher finished the lesson, they wrote the homework assignment down. Then you can do what you want, whether that’s for 20 seconds or 5 minutes.</p>