Consider an incomplete pyramid of balls on a square base having 18 layers and having 13 balls on each side of the top layer then the total number N of balls in that pyramid satifies-(a) 900010000(b) 80009000(c) 70008000(d) 1000012000

Top layer has 13*13 balls.
Similarly one layer below the top layer will have 14*14 balls.

We have 18 layers. So

N = 13^2+14^2+15^2+........+30^2

sum of squares of no. ranging from 18 to30 = sum of square of 30no.s -square of 12 no.s.Apply th formula n(n+1)2n+1)/6substitute 30 and 12 as n and subtract to get 8805