Sometimes, you make mistakes. I don’t care who you are, what your GPA is, or what your SAT scores are. Sometimes, you make mistakes. If you’re the kind of student who is able to finish sections before time is called, it’s pure hubris not to use that opportunity to check your work.

An anecdote: yesterday morning, I did a bunch of work typing up an explanation for the Q&A page, but then accidentally closed my browser and lost all my work before posting it. I was late for a train, so I didn’t have time to retype it all. I figured that once I got to campus—school has started again for me, too—I’d have a chance to retype it. And I did.

For the first time that I’m aware of, I posted an incorrect answer on my Q&A. And it’s not that I didn’t know how to do the question. I did all the “hard” math correctly. But when it came time to add 1 + 5 + 10 + 10 + 5 + 1, I got 22 instead of 32. And I posted that. It was up for about 18 hours before I woke up today, half-dazed, and muttered wait…22 isn’t a power of 2! (The super-fast solution I hinted at in that post is that the number of subsets of a set with n elements is 2n.) I got out of bed, changed one digit in a post, and went back to bed.

So what’s the point? I’m good at math. I’m sure-footed, especially when it comes to SAT-style problems. I don’t second guess myself. The fact that you’re reading this means that I have a reputation for being an authority on the topic. And still, I managed to make a very silly mental math error, the kind that costs people dearly on tests. And I posted it without checking my work. It was the first time in nearly a year of running the Q&A, so it doesn’t happen often, but I made a silly mistake and didn’t catch it. Sometimes, everyone makes mistakes.

Maybe some readers saw that I did all the work right, and just assumed it was a typo. That’s generous, and I thank them. But you know who won’t be as generous? You know who doesn’t care whether you did all the work right? You guessed it: the SAT. So do as I say, not as I failed to do yesterday.

Check your work if you have time.

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