Puzzle 58: The Number Above

See how the whole numbers are arranged.  For example, 18 is above 13, 12 is above 8.  If you keep writing number this way, eventually you will have the row that contains 100 and the row above.  However, is there a way to find what is the number above 100 without writing all the numbers?  What about the number above 1000?  Explain you method(s).

Extension: How can you know instantly what is the number above a number, n?

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