Chapter P, Prerequisites - Section P.9 - Modeling with Equations - P.9 Exercises - Page 72: 61

$500mph$

Let $v$ be the outwards average speed and $t$ the outwards trip time. Then $1.2v$ be is the return average speed and $\frac{55}{6}-t$ the return trip time. Then our equations: $vt=2500$ and $1.2v(\frac{55}{6}-t)=2500\\11v-1.2vt=2500\\11v-1.2\cdot2500=2500\\11v=5500\\v=500$