1 what is the error in this? I can't get it...
2
3
4
5
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20

... I'm going to need more info than that... is this a mathematical brainteaser..

If it's just a straight up riddle, I'm going to go with...

1. the sentence "what is the error in this?" shouldn't be next to the 1... it should be above it.
2. it stops at 20. Why does it stop at 20?

I'm assuming people are going to look for a "missing number" in which case, there isn't.

The apostrophe in can't is missing?