Avoid Arc Elasticities
Avoid the midpoint-method of calculating percentages. Avoid arc elasticities. In my view, these methods are a distracting waste of time that make it difficult to understand the important concept of elasticity. I NEVER use them in the course or on exams.
Unfortunately, the midpoint method is used for arc elasticities of demand in the Minnesota and Openstax Textbooks (Ch 5, Sec 1 in both books). For that reason, I do not suggest either section as a reference reading. If you see these methods being used in outside readings, textbooks or in review problems, please avoid reading them. And do not memorize the arc-elasticity formula. Memorizing complicated formulas is almost always a bad idea.