Answer Key 7.10

[latexpage]

\(\begin{array}{rrl}
\text{2012 average age}&=&\phantom{-}\text{2016 average age} \\ \\
10x&=&\phantom{-}9(x+4)+N \\
10x&=&\phantom{-}9x+36+N \\
-9x&&-9x \\
\midrule
x&=&\phantom{-}36+N \\ \\
x&=&\phantom{-}36+N \\
-36&&-36 \\
\midrule
x-36&=&\phantom{-}N
\end{array}\)

This means the new member is 36 years younger than the average age in 2012.