Let's boil it down to an absurd level. Imagine we are measuring the economy of Koeppia-Weaverstan, a new nation of two brilliant economists and no one else. Imagine I pay you to mow my lawn. On the expenditure side of measuring GDP, the $20 I give you is found. On the income side, that same $20 is reflected in your income. The accounting is different, but they are measuring the same thing - the two approaches should be equal.
Chapter 7, 8, & 9 Curiosities
GDP
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