tex file to explain facebook algbra puzzle

macdonalds.tex

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+\documentclass[a4paper,10pt]{article}
+\usepackage[utf8]{inputenc}
+
+%opening
+\title{}
+\author{}
+
+\begin{document}
+
+\maketitle
+
+\begin{abstract}
+
+\end{abstract}
+
+\section{McDonalds maths puzzle etc.}
+
+Where $D$ is a drink, $B$ is a burger and $F$ is fries:
+
+$$ D + D + D = 30 $$
+so $D$ == 10
+$$ D + B + B = 20 $$
+so $B$ == 5
+$$ B + FF + FF = 9 $$
+so $FF + FF = 4$ therefore $FF=2$ so $F=\pm \sqrt{2}$
+
+Onto the final equation
+$$B + F \times D = \; ? $$
+
+As $F$ is  $\pm \sqrt{2}$ 
+
+there are two valid answers
+
+$ 5 + \sqrt{2} \times 10 $ and $ 5 - \sqrt{2} \times 10 $
+
+So the answer is either 19.142 or -9.142.
+
+
+\end{document}