Remember that $3\times 10$ means $3$ groups of $10.$ Therefore,

\begin{align*} {\color{blue}{3}}\times 10 &=\\[5pt] \underbrace{10 + 10 + 10}_{{\color{blue}{3}}\,\textrm{times}} &=\\[5pt] {\color{blue}{3}}\,\textrm{tens} + 0\,\textrm{ones}&=\\[5pt] {\color{blue}{3}}0.& \end{align*}

So, $ 3\times 10 = \bbox[3pt,Gainsboro]{\color{blue}30}. $

Remember that $3\times 10$ means $3$ groups of $10.$ Therefore,

\begin{align*} {\color{blue}{3}}\times 10 &=\\[5pt] \underbrace{10 + 10 + 10}_{{\color{blue}{3}}\,\textrm{times}} &=\\[5pt] {\color{blue}{3}}\,\textrm{tens} + 0\,\textrm{ones}&=\\[5pt] {\color{blue}{3}}0.& \end{align*}

Remember that $12\times 10$ means $12$ groups of $10.$ Therefore,

\begin{align*} {\color{red}{1}}{\color{blue}{2}}\times 10 &=\\[5pt] \underbrace{10 + 10 + \cdots +10}_{{\color{red}{1}}{\color{blue}{2}}\, \textrm{times}} &=\\[5pt] {\color{red}{1}}{\color{blue}{2}}\,\textrm{tens} + 0\,\textrm{ones}&=\\[5pt] {\color{red}{1}}\,\textrm{hundreds} + {\color{blue}{2}}\,\textrm{tens} + 0\,\textrm{ones}&=\\[5pt] {\color{red}{1}}{\color{blue}{2}}0.& \end{align*}

Remember that $2\times 100$ means $2$ groups of $100.$ Therefore,

\begin{align*} {\color{blue}{2}}\times 100 &=\\[5pt] \underbrace{100 + 100}_{{\color{blue}{2}}\,\textrm{times}} &=\\[5pt] {\color{blue}{2}}\,\textrm{hundreds} + 0\,\textrm{tens} + 0\,\textrm{ones}&=\\[5pt] {\color{blue}{2}}00.& \end{align*}

Remember that $25 \times 100$ means $25$ groups of $100.$ Therefore, \begin{align*} {\color{red}{2}}{\color{blue}{5}} \times 100 & = \\[5pt] \underbrace{100 + 100 + \cdots + 100}_{{\color{red}{2}}{\color{blue}{5}} \, \textrm{times}} & = \\[5pt] {\color{red}{2}}{\color{blue}{5}}\,\textrm{hundreds} + 0\,\textrm{tens} + 0\,\textrm{ones} & = \\[5pt] {\color{red}{2}}\,\textrm{thousands} + {\color{blue}{5}}\,\textrm{hundreds} + 0\,\textrm{tens} + 0\,\textrm{ones} & = \\[5pt] {\color{red}{2}},{\color{blue}{5}}00.& \end{align*}

So, $25 \times 100 = \bbox[3pt,Gainsboro]{\color{blue}2,500}.$

Remember that $34\times 100$ means $34$ groups of $100.$ Therefore,

\begin{align*} {\color{red}{3}}{\color{blue}{4}}\times 100 &=\\[5pt] \underbrace{100 + 100 + \cdots + 100}_{{\color{red}{3}}{\color{blue}{4}}\,\textrm{times}} &=\\[5pt] {\color{red}{3}}{\color{blue}{4}}\,\textrm{hundreds} + 0\,\textrm{tens} + 0\,\textrm{ones}&=\\[5pt] {\color{red}{3}}\,\textrm{thousands} + {\color{blue}{4}}\,\textrm{hundreds} + 0\,\textrm{tens} + 0\,\textrm{ones}&=\\[5pt] {\color{red}{3}},{\color{blue}{4}}00.& \end{align*}

Remember that $9 \times 1,000$ means $9$ groups of $1,000.$ Therefore, \begin{align*} {\color{blue}{9}} \times 1,000 & =\\[5pt] \underbrace{1,000 + 1,000 + \cdots + 1,000}_{\color{blue}{9}\, \textrm{times}} & = \\[5pt] {\color{blue}{9}}\,\textrm{thousands} + 0\,\textrm{hundreds} + 0\,\textrm{tens} + 0\,\textrm{ones} & = \\[5pt] {\color{blue}{9}},000.& \end{align*}

Remember that $18\times 1,000$ means $18$ groups of $1,000.$ Therefore,

\begin{align*} {\color{red}{1}}{\color{blue}{8}}\times 1,000 &=\\[5pt] \underbrace{1,000 + 1,000 + \cdots + 1,000}_{{\color{red}{1}}{\color{blue}{8}}\, \textrm{times}} &=\\[5pt] {\color{red}{1}}{\color{blue}{8}}\,\textrm{thousands} + 0\,\textrm{hundreds} + 0\,\textrm{tens} + 0\,\textrm{ones}&=\\[5pt] {\color{red}{1}}\,\textrm{ten-thousands} + {\color{blue}{8}}\,\textrm{thousands} + 0\,\textrm{hundreds} + 0\,\textrm{tens} + 0\,\textrm{ones}&=\\[5pt] {\color{red}{1}}{\color{blue}{8}},000.& \end{align*}