Question

7,51,13,721,? missing number find
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Answer 1

Answer:

7

reverse number. 51=15, 13=31, 721=127

so, new series is;

7,15,31,127

7×2+1=15

15×2+1=31

31×2+1=63

63×2+1=127

63 reverse is 36,

so, 36 is missing.