The middle of the triangle contains the square root of the sum of the three surrounding numbers.
Think of words ending in -GRY. Angry and hungry are two of them. There are only three words in the English language. What is the third word? The word is something that everyone uses every day. If you have listened carefully, I have already told you what it is.
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Hint: Is this a trick question?
Answer: There are only two: angry and hungry. The rec.puzzles archive offers a large collection of words that end in -GRY, but none of them could be considered even remotely common. There are many generally unsatisfying "trick" answers to the problem, which depend on a specific wording of the question or that the question be spoken instead of written. There seems to be no agreement among puzzle historians about which form is the original, or even the age of the problem. In any event, it is apparent that the frequent mutations of the puzzle statement over the years have erased whatever answer was intended by the original author. The usual trick is to play on the expression "the English Language", you are then asked for the third word - which is of course Language!
...the 20th prime number?
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Hint: The prime numbers start with 2, 3, 5, 7, 11, 13, 17, 19, 23.
The initial letters are RWASEN, which is an anagram of ANSWER.