How does the paper computer work?
 
If you stack the cards in numerical order and look at them from the side, you'll see a pattern that looks something like this:
The "ones" can be seen in a pattern when the card is viewed from the side.

 
"Is your number on this disk?" "Yes"
Then keep all the cards that have a black mark in the left column (the 16's place)
"Is your number on this disk?"
"No"
Keep all the cards that do NOT have a black mark in the second from the left column (the 8's place).
"Is your number on this disk?"
"Yes"
Keep all the cards that have a black mark in the middle column (the 4's place)
"Is your number on this disk?"
"No"
Keep all the cards that do NOT have a black mark in the fourth column (the 2's place)
"Is your number on this disk?"
"Yes"
Keep the card that  has a black mark in the last column (the 1's place)
...
"Your number is 21."
"Wow!"



 


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