so let's take a peek at both's ages, keep in mind, every year, is 1year added to Irene and 1year added to Fred
so... if we look at their ages [tex]\bf \begin{array}{ccllll}
fred&irene\\
\textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\
58&18\\
57&17\\
56&16\\
55&15\\
...&...
\end{array}[/tex]
notice, Fred is always 40years older than Irene
thus, whatever age Irene is, let's say "i", then Fred is " i + 40 "
now, when is Fred 5 times Irene's age or 5*i or 5i? well,
f = fred's age i = irene's age
f = i + 40
now if f = 5i
5i = i + 40 <--- solve for "i" to see how old Irene was then
