We have a sequence of numbers :

\(12;21;30;39;48;B;66;...\)

The numbers separated by 9 units 

=> B = 48 + 9 = 57

So B = 57

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B là 57

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We have: 12; 21;30;39;48;B;66

We see: 21=12+9 ; 30=21+9; 39=30+9; 48=39+9; B=48+9; 66=B+9

=> B= 66-9 (or 48+9) = 57

So B=57