find the missing number 13, 10, 17, 15, 21, _, 25, 25
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3 Answers

start: 10, 15, ???, 25

anser: 10, 15, 20, 25
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10-4=9 write the missing number
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It is progressively subtracting/adding smaller and smaller numbers.

so 13 - 3(10) + 7(17) and then 17-2(15) + 6(21)

and it then gets to the point where it becomes minus 0 so 25 - 0 = 25. and since that 25 is minus 0, the number before would have to have 1 subtracted, so the answer is 20.
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