Column 7: The Back of the Envelope

It was in the middle of a fascinating conversation on software engineering that Bob Martin asked me, “How much water flows out of the Mississippi River in a day?” Because I had found his comments up to that point deeply insightful, I politely stifled my true response and said, “Pardon me?” When he asked again I realized that I had no choice but to humor the poor fellow, who had obviously cracked under the pressures of running a large software shop.

My response went something like this. I figured that near its mouth the river was about a mile wide and maybe twenty feet deep (or about one two-hundred-and-fiftieth of a mile). I guessed that the rate of flow was five miles an hour, or a hundred and twenty miles ...

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