Answer Key 7.10

[latex]\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 \\ \hline x&=&\phantom{-}36+N \\ \\ x&=&\phantom{-}36+N \\ -36&&-36 \\ \hline x-36&=&\phantom{-}N \end{array}[/latex]

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