made it generic with macros.

facebook_maths_picture_puzzle.tex

@@ -1,5 +1,12 @@
 \documentclass[a4paper,10pt]{article}
 \usepackage[utf8]{inputenc}
+\newcommand{\theme}{Macdondalds}
+\newcommand{\ttt}{D}
+\newcommand{\topname}{Drink}
+\newcommand{\mmm}{M}
+\newcommand{\midname}{Burger}
+\newcommand{\bbb}{F}
+\newcommand{\botname}{fries}
 
 %opening
 \title{}
@@ -10,31 +17,29 @@
 \maketitle
 
 \begin{abstract}
-
+This small document un~picks the common
+facebook maths picture puzzle. % solution
 \end{abstract}
 
-\section{Various themes but always the same hidden multiply: McDonalds maths puzzle etc.}
+\section{Various themes but always the same hidden multiply: this one is the {\theme} maths puzzle etc.}
 
-Where $D$ is a drink, $B$ is a burger and $F$ is fries:
+Where $\ttt$ is a {\topname}, $\mmm$ is a {\midname} and $\bbb$ is {\botname}:
 
-$$ D + D + D = 30 $$
-so $D$ == 10
-$$ D + B + B = 20 $$
-so $B$ == 5
-$$ B + FF + FF = 9 $$
-so $FF + FF = 4$ therefore $FF=2$ so $F=\pm \sqrt{2}$
+$$ \ttt + \ttt + \ttt = 30 $$
+so $\ttt$ == 10
+$$ \ttt + \mmm + \mmm = 20 $$
+so $\mmm$ == 5
+$$ \mmm + \bbb \bbb + \bbb \bbb = 9 $$
+so $\bbb \bbb + \bbb \bbb = 4$ therefore $\bbb \bbb=2$ so $\bbb=\pm \sqrt{2}$
 
 Onto the final equation
-$$B + F \times D = \; ? $$
-
-As $F$ is  $\pm \sqrt{2}$ 
-
-there are two valid answers
+$$\mmm + \bbb \times \ttt = \; ? $$
 
+As $\bbb$ is  $\pm \sqrt{2}$  there are actually two valid answers:
 $ 5 + \sqrt{2} \times 10 $ and $ 5 - \sqrt{2} \times 10 $
-
 So the answer is either 19.142 or -9.142.
 
-
+\vspace{0.5cm}
+typeset in {\LaTeX} on {\today}.
 \end{document}