5. The parabola \(C\) has equation \(y ^ { 2 } = 20 x\).
- Verify that the point \(P \left( 5 t ^ { 2 } , 10 t \right)\) is a general point on \(C\).
The point \(A\) on \(C\) has parameter \(t = 4\).
The line \(l\) passes through \(A\) and also passes through the focus of \(C\). - Find the gradient of \(l\).