# Aligning matrices with enumerate

I am trying to align matrices in three questions like an array but I have an enumerate list. If I create the array then the lists won't show.

\item $$\begin{pmatrix*}[r] 20 & 25 & 20 \\ 15 & 20 & 18 \\ 18 & 23 & 22 \end{pmatrix*}$$
$$\begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*}$$ =
$$\begin{pmatrix*}[r] 975 \\ 810 \\ 965 \end{pmatrix*}$$
$$\qquad\qquad\Rightarrow\begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*}$$ =
$$\begin{pmatrix*}[r] 10 \\ 15 \\ 20 \end{pmatrix*}$$
\item $$\begin{pmatrix*}[r] 90 & 95 & 80 \\ 80 & 85 & 90 \\ 75 & 90 & 95 \end{pmatrix*}$$
$$\begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*}$$ =
$$\begin{pmatrix*}[r] 86 \\ 86 \\ 88 \end{pmatrix*}$$
$$\qquad\qquad\Rightarrow\begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*}$$ =
$$\begin{pmatrix*}[r] 0.3 \\ 0.2 \\ 0.5 \end{pmatrix*}$$
\item $$\begin{pmatrix*}[r] 5,000 & 4,000 \\ 6,000 & 5,000 \end{pmatrix*}$$
$$\begin{pmatrix*}[r] x \\ y \\ \end{pmatrix*}$$ =
$$\begin{pmatrix*}[r] 37,000,000 \\ 45,000,000 \\ \end{pmatrix*}$$
$$\qquad\qquad\Rightarrow\begin{pmatrix*}[r] x \\ y \\ \end{pmatrix*}$$ =
$$\begin{pmatrix*}[r] 5000 \\ 3000 \end{pmatrix*}$$

• (1) welcome, (2) please provide a full compilable example that others can copy and test without having to add anything. Here for example, one needs to know that mathtools is needed – daleif Oct 6 '16 at 7:57
• You'll probably have to make a box on each manually to align the \Rightarrows – daleif Oct 6 '16 at 7:59

This is an automatic solution, based on the eqparbox package. I defined an \eqmathbox command, which accepts $2$ optional arguments (the end of name of the box, and the alignment: l, r or c) and one mandatory argument: the contents of the math box.

\documentclass{article}
%\usepackage{enumitem}
\usepackage{mathtools}
\usepackage{xparse}
\usepackage{eqparbox}
\NewDocumentCommand\Eqmathbox{O{0}O{l}m}{\eqmakebox[Form#1][#2]{\ensuremath{ \displaystyle#3}}}
\usepackage{siunitx}
\sisetup{group-separator={,},group-minimum-digits = 4}

\begin{document}

\begin{enumerate}
\item $$\Eqmathbox{\begin{pmatrix*}[r] 20 & 25 & 20 \\ 15 & 20 & 18 \\ 18 & 23 & 22 \end{pmatrix*} \begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*} = \begin{pmatrix*}[r] 975 \\ 810 \\ 965 \end{pmatrix*}} \Rightarrow\Eqmathbox[1][r]{\begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*} =} \begin{pmatrix*}[r] 10 \\ 15 \\ 20 \end{pmatrix*}$$
\item $$\Eqmathbox{\begin{pmatrix*}[r] 90 & 95 & 80 \\ 80 & 85 & 90 \\ 75 & 90 & 95 \end{pmatrix*} \begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*} = \begin{pmatrix*}[r] 86 \\ 86 \\ 88 \end{pmatrix*}} \Rightarrow\Eqmathbox[1][r]{\begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*} =} \begin{pmatrix*}[r] 0.3 \\ 0.2 \\ 0.5 \end{pmatrix*}$$
\item $$\Eqmathbox{\begin{pmatrix*}[r] \num{5000} & \num{400}0 \\ \num{6000} & \num{5000} \end{pmatrix*} \begin{pmatrix*}[r] x \\ y \\ \end{pmatrix*} = \begin{pmatrix*}[r] \num{37000000} \\ \num{45000000 }\\ \end{pmatrix*}} \Rightarrow\Eqmathbox[1][r]{\begin{pmatrix*}[r] x \\ y \\ \end{pmatrix*} =} \begin{pmatrix*}[r] \num{5000} \\ \num{3000} \end{pmatrix*}$$
\end{enumerate}

\end{document}


Here is one way. Note that you are adding a lot of unneeded $$...$$-pairs. I've removed them. The only reason memoir is used, is because it is the default in my editor.

\documentclass[a4paper]{memoir}
\usepackage{mathtools}
\begin{document}

\begin{enumerate}
\item $$\mathmakebox[6cm][l]{ \begin{pmatrix*}[r] 20 & 25 & 20 \\ 15 & 20 & 18 \\ 18 & 23 & 22 \end{pmatrix*} \begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*} = \begin{pmatrix*}[r] 975 \\ 810 \\ 965 \end{pmatrix*} } \qquad\qquad \Rightarrow\begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*} = \begin{pmatrix*}[r] 10 \\ 15 \\ 20 \end{pmatrix*}$$
\item $$\mathmakebox[6cm][l]{ \begin{pmatrix*}[r] 90 & 95 & 80 \\ 80 & 85 & 90 \\ 75 & 90 & 95 \end{pmatrix*} \begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*} = \begin{pmatrix*}[r] 86 \\ 86 \\ 88 \end{pmatrix*} } \qquad\qquad\Rightarrow\begin{pmatrix*}[r] x \\ y \\ z \end{pmatrix*} = \begin{pmatrix*}[r] 0.3 \\ 0.2 \\ 0.5 \end{pmatrix*}$$
\item $$\mathmakebox[6cm][l]{ \begin{pmatrix*}[r] 5,000 & 4,000 \\ 6,000 & 5,000 \end{pmatrix*} \begin{pmatrix*}[r] x \\ y \\ \end{pmatrix*} = \begin{pmatrix*}[r] 37,000,000 \\ 45,000,000 \\ \end{pmatrix*} } \qquad\qquad\Rightarrow\begin{pmatrix*}[r] x \\ y \\ \end{pmatrix*} = \begin{pmatrix*}[r] 5000 \\ 3000 \end{pmatrix*}$$
\end{enumerate}

\end{document}