Q: Example of Box inside Box I would like to create box inside box as the picture below :

Here is my code :
\documentclass[10pt,a4paper]{report}
\usepackage[margin=0.5in]{geometry}
\usepackage{amsthm,amssymb,amsfonts}
\usepackage{tikz,lipsum,lmodern}
\usepackage[most]{tcolorbox}
\usepackage[svgnames]{xcolor}

\newtcolorbox{Box1}[2][]{sidebyside,
                lower separated=false,
                colback=white,
colframe=white!20!gray,fonttitle=\bfseries,
colbacktitle=white!10!gray,enhanced,
attach boxed title to top left={xshift=1cm,
        yshift=-2mm},
title=#2,#1}

\newtcolorbox{Box2}[2][]{sidebyside,
                lower separated=false,
                colback=white!60!gray,
colframe=white!20!gray,fonttitle=\bfseries,
colbacktitle=white!10!gray,enhanced,
attach boxed title to top left={xshift=1cm,
        yshift=-2mm},
title=#2,#1}



\begin{document}

\begin{Box1}{Exemple}
 \begin{minipage}[b]{0.9\linewidth}\centering
\begin{tcolorbox}[colback=white!60!gray,colframe=white!20!gray]
On Considère une va $X$ à valeurs dans $\{1,2,3\}$ et de loi: $P\left(X=1\right)=\dfrac{1}{6}$, $P\left(X=2\right)=\dfrac{1}{3}$ $P\left(X=3\right)=\dfrac{1}{2}$.

Calculer $E\left(X\right)$ et $E\left(X^{3}\right)$ 
\end{tcolorbox}
\end{minipage}
\tcblower
On a : $E\left(X \right)=\sum_{k=1}^{3}kP\left(X=k\right)=1\dfrac{1}{6}+2\dfrac{1}{3}+3\dfrac{1}{2}=\dfrac{7}{3}$.
D'après la formule de transfert : 
\[E\left(X^3\right)=\sum_{k=1}^{3}P\left(X=k \right)=1^{3}\dfrac{1}{6}+2^{3}\dfrac{1}{3}+3^{3}\dfrac{1}{2}=\dfrac{49}{3} \]
\end{Box1}

\begin{Box2}{Méthode}
 \begin{minipage}[b]{0.6\linewidth}\centering
\begin{tcolorbox}[colback=white,colframe=white!20!gray]
Pour calculer la variance $V\left( X\right)$ d'une va $X$
\end{tcolorbox}
\end{minipage}
\tcblower
Essayer de :
\noident 
\begin{itemize}
\item Utiliser la formule : $V\left( X\right)=E\biggl(\left(X-E(x)\right)^{2}\biggr) $
\item Utiliser la formule : $V\left( X\right)=E\left( X^2\right)-\left(E\left( X^2\right)\right)^{2} $
\item Utiliser la formule : $V\left( X\right)=a^{2}V\left(Y\right)$ si $X=aY+b$
\end{itemize}
Exercice $28.1$ à $28.5$
\end{Box2}
\end{document}

which produce :



*

*Could someone please improve my code to create the same picture as i post on top 


A: You can use righthand ratio=<fraction> key to define right-handed part to the given <fraction> or righthand width=<width> for a fixed length 
Key sidebyside align=top seam to align left side box to the top 
\documentclass[10pt,a4paper]{report}
\usepackage[margin=0.5in]{geometry}
\usepackage{amsthm,amssymb,amsfonts}
\usepackage{tikz,lipsum,lmodern}
\usepackage[most]{tcolorbox}

\newtcolorbox{Box1}[2][]{sidebyside,
                lower separated=false,
                righthand ratio=0.56, % to define right-hand fraction
                colback=white,
colframe=white!20!gray,fonttitle=\bfseries,
colbacktitle=white!10!gray,enhanced,
attach boxed title to top left={xshift=1cm,
        yshift=-2mm},
title=#2,#1}

\newtcolorbox{Box2}[2][]{sidebyside,
                lower separated=false,
                righthand ratio=0.56,
                colback=white!60!gray,
colframe=white!20!gray,fonttitle=\bfseries,
colbacktitle=white!10!gray,enhanced,
attach boxed title to top left={xshift=1cm,
        yshift=-2mm},
title=#2,#1}



\begin{document}

\begin{Box1}{Exemple}
\begin{tcolorbox}[colback=white!60!gray,colframe=white!20!gray]
On Considère une va $X$ à valeurs dans $\{1,2,3\}$ et de loi: $P\left(X=1\right)=\dfrac{1}{6}$, $P\left(X=2\right)=\dfrac{1}{3}$ $P\left(X=3\right)=\dfrac{1}{2}$.

Calculer $E\left(X\right)$ et $E\left(X^{3}\right)$ 
\end{tcolorbox}
\tcblower
On a : $E\left(X \right)=\sum_{k=1}^{3}kP\left(X=k\right)=1\dfrac{1}{6}+2\dfrac{1}{3}+3\dfrac{1}{2}=\dfrac{7}{3}$.
D'après la formule de transfert : 
\[E\left(X^3\right)=\sum_{k=1}^{3}P\left(X=k \right)=1^{3}\dfrac{1}{6}+2^{3}\dfrac{1}{3}+3^{3}\dfrac{1}{2}=\dfrac{49}{3} \]
\end{Box1}

\begin{Box2}[sidebyside align=top seam]{Méthode}
\begin{tcolorbox}[colback=white,colframe=white!20!gray,width=.8\linewidth]
Pour calculer la variance $V\left( X\right)$ d'une va $X$
\end{tcolorbox}
\tcblower
Essayer de :
\noindent 
\begin{itemize}
\item Utiliser la formule : $V\left( X\right)=E\biggl(\left(X-E(x)\right)^{2}\biggr) $
\item Utiliser la formule : $V\left( X\right)=E\left( X^2\right)-\left(E\left( X^2\right)\right)^{2} $
\item Utiliser la formule : $V\left( X\right)=a^{2}V\left(Y\right)$ si $X=aY+b$
\end{itemize}
\mbox{}\hfill  Exercice $28.1$ à $28.5$
\end{Box2}

\end{document}

