in reply to (OT?) n(n-1)/2
How about:
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What is 1 + 1000?
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What is 2 + 999?
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What is 3 + 998?
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How many pairs of numbers adding up to 1001 are there from 1 to 1000? Are there any leftover numbers?
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What, then, is the sum of all the numbers from 1 to 1000?
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What happens if we replace 1000 with 100, 999 with 99, etc.?
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What happens if we replace 1000 with 1001, 999 with 1000, etc.?