Using the shorter method, simplify the given one.

$6135\times 999$

As we know, $999$ can be written as $(1000-1)$ so,

$$\begin{gathered} =6135\times (1000-1) \\ =6135\times 1000-6135\times 1 \\ =6135000-6135 \\ =\mathbf{6128865} \end{gathered}$$

Hence, the required answer is$\mathbf{6128865}$.