Answer
$56 \text{ miles per hour}$
Work Step by Step
The round trip distance is
\begin{array}{l}\require{cancel}
2(154)
\\\\=
308\text{ miles}
.\end{array}
Using $D=rt,$ or the relationship of distance, rate, and time, then the average speed is
\begin{array}{l}\require{cancel}
D=rt
\\\\
308=r\left(5\dfrac{1}{2}\right)
\\\\
308=r\left(\dfrac{11}{2}\right)
\\\\
308=\dfrac{11}{2}r
\\\\
\dfrac{2}{11}\cdot308=\dfrac{11}{2}r\cdot\dfrac{2}{11}
\\\\
\dfrac{616}{11}=r
\\\\
r=56
.\end{array}
Hence, the average speed is $
56 \text{ miles per hour}
.$
