In this scenario, we have to use an inverse rule of three:
[tex]\begin{gathered} 70mph\rightarrow3.5h \\ 80\text{mph}\rightarrow x\text{ hours} \\ \Rightarrow x=\frac{(70mph)(3.5h)}{80\text{mph}}=3.0625\text{ hours} \end{gathered}[/tex]Therefore, the trip back took approximately 3.1 hours
