Find the missing term : 

1 BR, 2 EO, 6 HL, 15 KI, ? 
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1 Answers

The first number in the terms follow the sequence +12, +22, +32, +42, ........ The second letter of each group differs by 3 letters in the forward direction. Third letter of each group differs by 3 letters in the backward direction. Hence, the next choice would be 31NF. 

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