3,22,7,45,15?,31

• 3*2 +1 =7
  22*2 +1=45
  7*2 + =15
  45*2 +1 =91
  15*2 +1=31
  
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• ans is 91
  3+4=7,22+23=45,7+8=15,45+46=91,15+16=31
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• series starting from 1st element
  
  difference
  7-3= 4
  15-7= 8
  31-15= 16 Each difference is multiplied by 2
  
  series starting from 2nd element
  
  difference
  45-22
  =23
  So multiply 23 by 2(ans is 46)
  Now add 45+46 =91
  
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