The number of natural numbers less than one million that can be formed by using the digits, $0, 2$, and 3 is

The number of k digit natural numbers formed by using the digit  and 3 is $2\left(3^{k-1}\right)$

$\therefore$ Number of natural numbers less than one million formed by $0,2$ and 3 is

$\begin{array}{l}=2+\left(2\right)\left(3\right)+\left(2\right)\left(3^2\right)+2\left(3^3\right)+2\left(3^4\right)+2\left(3^5\right)\\ =728\end{array}$