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I think you are missing the point. No simple math tricks can overcome the fact that I am impressed at the amount on my credit card bill at the end of the month.

What the author tried to convey is more the incapacity of the brain to apprehend a large amount of small numbers.

Simple math will tell you to estimate your credit card bill using old credit card bills as a reference, not your gut feeling about how much you spend.

I simply noticed that the bottom two rows had numbers clustered around 30. So I did 30 * 20 = 600. Not as close as your guess, but close enough :)

Yeah, same here. I noticed a few spots were blank, so I'd say around 540, but yeah, close enough.

Same here... I counted 14 boxes with numbers around 30, so that was 420. I rounded up to 500 because I knew there were skipped boxes. All under 5 seconds.