We can check it through option. If x is the number of children in 1 st row. then in 2nd row = (x - 3) children in 3rd row = (x - 6) children ................ ...................... 6th row = (x -15) children ∴x+x−3+x−6+x−9+x−12+x−15=630 ⇒6x−45=630 ⇒6x=675 ⇒n=
675
6
=112.5 So, 6 row is not possible as number of children cannot be in fraction.