An architect designs a modern building featuring a large glass window pane in the shape of a trapezium \(PQRS\), where \(PQ \parallel SR\). The non-parallel sides are \(PS\) and \(QR\). To make the structure robust, metal brackets are welded at the vertices. The design blueprint lists the internal angles of the window pane as \(\angle P = (5x + 40)^{\circ}\) and \(\angle S = (5x - 30)^{\circ}\).
1. Which geometric property allows us to find the value of \(x\)?
2. The value of \(x\) is
3. If \(\angle Q = 78^{\circ}\), then \(\angle R\) is
4. What is the primary condition for this trapezium \(PQRS\) to be classified as an "isosceles trapezium"?