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a> Sum = 4×3 + 5×3 + 6×3 + 7×3 + 8×3 + 9×3… + 31×3 + 32×3 + 33×3

               = {4+5+6+(7+33)x[(33-7)÷2+1]}x3

               = {15+40×14}x3

               = (15+560)x3

               = 575×3

               = 1735

b> There are 33-4+1 = 30 numbers

     for 3 in ones: only 11×3, 21×3, 31×3

     for 3 in tens: 10×3, 11×3, 12×3, 13×3

     total there are 6 with 3 digits. 

     Therefore, there are 30- 6 = 24 numbers do not contain digit 3.

 

 

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