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 satisfies

The top layer has $(13\times 13)$ balls the layer below it  will have $(14\times 14)$ balls

We have $18$ layers

So the total number of balls 

$$\begin{gathered} N=(13\times 13)+(14\times 14)+.......(30\times 30) \\ N={13}^2+{14}^2+.....{30}^2 \end{gathered}$$

Sum of squares of first $n$ natural numbers is $\frac{n(n+1)(2n+1)}{6}$

$\therefore N=$ sum of first $30$ $-$ sum of first $12$

$$\begin{gathered} N=\frac{30\times 31\times 61}{6}-\frac{12\times 13\times 25}{6} \\ N=8805 \end{gathered}$$

$\Rightarrow 8000<N<9000$