In $\triangle PQR$,$\angle P = 70^{\circ}$ and $\angle R = 30^{\circ}$. Which side of this triangle is the longest? Give a reason for your answer.

(PR) In $\triangle PQR$,the sum of all interior angles is $180^{\circ}$.
$\angle Q = 180^{\circ} - (\angle P + \angle R)$
$\angle Q = 180^{\circ} - (70^{\circ} + 30^{\circ}) = 180^{\circ} - 100^{\circ} = 80^{\circ}$.
Comparing the angles,we have $\angle Q > \angle P > \angle R$ $(80^{\circ} > 70^{\circ} > 30^{\circ})$.
According to the property of triangles,the side opposite to the largest angle is the longest side.
The angle $\angle Q$ is the largest angle,and the side opposite to $\angle Q$ is $PR$.
Therefore,$PR$ is the longest side of $\triangle PQR$.