Question

15,21,57_,221answermethods

Answer 1

This series is like 1,5,21,57,..........,221
there is difference of 4, 16, 36 which are squres of 2,4,6 respectively.
So next difference will be square of 8=64 so 57+64=121 is the missing number in the series. 1,5,21,57,121,221.

Hope its help
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