Dear student,
First of all generate the AP and then take the sum






















































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Sum of nos. not divisible by 6 and below 400 = {sum of all nos. below 400} - {sum of all nos. divisible by 6 and below 400}
= {1+2+........+399} - {6+12+18+..........+396}
= {399/2[1+399]} - {66/2[6+396]}
= {399*200} - {33*402}
= {79800}-{13226}
= 66574
you can solve this and will get the answer.