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The number 10 comes between 6 and 15. The numbers 3, 6, 10, 15, 21, and 28 are called TRIANGLE NUMBERS because they can be represented by lines of dots that form an equilateral triangle. They represent the additive sum 1+2+...+n which is equal to n (n+1)/2.

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Q: What number is missing from the sequence 3 6 15 21?
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What is the missing number in this sequence 17-23- -18-20-25-21-9-15-16?

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