9 student  can seat in (9-1)! way as 1 boy is taken in a refrence
therfor 8!=8*7*6*5*4*3*2*1=40320ways
if anti colk or clok wise direction detemi then ans is 40320/2=20160
but it is not detemi so ans is=40320

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You have to have a idea of solving sums of permutation to do this type of sums we need  to take i thing as fixed and the the must be arranged so here (9 - 1)! = 
                                                                  8! = 8 × 7 × 6 × 5 × 4 × 3 × 2 × 1
                                                                       = 40320 ways
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