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The steel door appears to be held shut by an electronic lock. You suspect it to be jammed, its power source run down long ago, but as soon as you run your fingers over the keypad, an LED display glows into life above it.

Eleven numbers appear, arranged in three horizontal rows. There are four numbers on the top row, four on the second, and three on the third:

You study the rows of numbers carefully, looking for a pattern. By tapping on the keypad the number that is missing from the last row, you will release the lock and open the door.

When you think you know the missing number turn to the entry that bears that number.⁹

If you cannot work out the correct answer, turn to 194.