Ans: $1.75$ hours
Let $x \text{hours}$ be the time needed from city $P$ to city $Q$.
Let $x \text{hours}$ be the time needed from city $P$ to city $Q$.
$\begin{array}{rcl}
\text{distance from $P$ to $Q$} + \text{distance from $Q$ to $R$} & = & \text{total distance} \\
72 \times x + 90 \times (\dfrac{161}{60}-x) & = & 210 \\
-18 x + 241.5 & = & 210 \\
-18x & = &-31.5 \\
x & = & 1.75
\end{array}$
Therefore, the car takes $1.75$ hours to travel from city $P$ to city $Q$.
