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puzzle.tex

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+\documentclass[a4paper,10pt]{article}
+\title{A mother is 21 years older than the child. 6 years from now the mother will be 5 times as old as the child.
+Question: Where is the father ? }
+\author{A. Puzzle}
+\begin{document}
+\maketitle
+\begin{abstract}
+Spoiler alert ! Do not read if you want to work it out yourself.
+\end{abstract}
+answer overleaf...
+\clearpage
+\paragraph{The answer....}
+Mother is 21 years older than child so, using $m$ for the age of the mother and $c$
+for the age of the child.
+$$  c + 21 = m $$
+\begin{equation}
+\label{s1}
+  c = m - 21 
+\end{equation}
+In 6 years time the mother is 5 times older than the child SO...
+$$  5(c+6) = m + 6 $$
+So
+$$  5c + 30 = m + 6 $$
+\begin{equation}
+\label{s2}
+  5c = m - 24
+\end{equation}
+Subtracting Equation (\ref{s1}) {\em from}  Equation (\ref{s2}) gives:
+$$ 5c - c  = m - m -24 - (-21) $$
+$$ 4c = -3 $$
+$$  c = -3/4 $$
+The baby is therefore -0.75 years old at present.
+which is -9 months. So the Baby is being conceived, so we know where the father is afterall! 
+\end{document}