Chapter 1, Equations and Graphs - Section 1.4 - Solving Quadratic Equations - 1.4 Exercises - Page 124: 79

$240mph$

Let $v$ be the speed between A and B and let that journey take $t$ time in hours. Then our equations: $vt=120$ and $(v+10)(t+0.1)=150\\vt+10t+0.1v+1=150\\120+10t+0.1v=149\\10t+0.1v=29$ $t=\frac{120}{v}$, thus $\frac{1200}{v}+0.1v=29\\v^2-290v+12000=0\\(v-145)^2=9025\\v-145=95\\v=240$