[omgwtf1000]

Here’s one of my favorites - super easy problem that almost everybody gets wrong.

On a piece of paper, write the following numbers as a vertical column:

4000

  40

  20

  30

  10

For best results, 4000 should be first and 10 last. Now take a second piece of paper and cover the number list. Ask someone to add the numbers up out loud as you move down the list, uncovering one number at a time starting with 4000. (First you expose the 4000, next the 40, and so on.) In nearly every case the person will call out the following: 4000, 4040, 4060, 4090, 5000. And typically they won’t believe you when you tell them they’re wrong, sometimes even after giving them the paper to redo their math.

Edit: formatting