5. What number replace the question mark : 100 99 96 91 84 75 64 ?

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What number replace the question mark : 100 99 96 91 84 75 64 ?

100 → 99 (−1) → 96 (−3) → 91 (−5) → 84 (−7) → 75 (−9) → 64 (−11)

The subtractions are odd numbers increasing by 2: 1, 3, 5, 7, 9, 11… next is 13.

So next term = 64 − 13 = 51.