Simply follow these instructions:
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Pick a positive integer n that is bigger than 1 but still fairly small and suits you.
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Legibly write the integers from 1 to n(n+3)/2 either in a column if that many won't fit in a row or else in a row if they will.
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Repeat n times: Cross off your choice of EITHER the first uncrossed number from the beginning of the row or column OR the first uncrossed number from the end.
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Sum the numbers you crossed out, but use a calculator because this trick isn't something where obnoxious mental calculation is something you should bother with.
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Divide this sum by n(n+1)/2 to get a new number which we will call "the".
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Start at the first uncrossed number in your row/column (as 1) and count numbers in the row/column until you get to the "the"th number.
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Subtract n from the number you just stopped counting on, but do it in your head this time because typing into the calculator would just be a waste of effort for this one.
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Read the last words in each of instruction 1 through 7 for my prediction.