To obtain the sum of successive odd digits, count the number of odd digits and square this number. The example 1+3+5+7+9+11 has 6 digits, and squaring this number yields the sum of the digits, which is 6×6=36

This is easy: 1+3=4 1+3+5=9 1+3+5+7=16 1+3+5+7+9=25

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