The given parabola y2=16x is of the form y2=4ax, hence a=4 and the focus of the parabola is (a,0)=(4,0).

The equation of a line joining the points (x1,y1) and (x2,y2) is y−y1=(x2−x1y2−y1)(x−x1).
Therefore, equation of chord joining P(1,4) to focus S(4,0) is
y−0=−34(x−4)
⇒3y=−4x+16
⇒4x+3y−16=0
To find the points where this line will cut the parabola put x=16y2 from the parabola into the line, to get
4(16y2)+3y−16=0
⇒y2+12y−64=0
⇒(y+16)(y−4)=0
⇒y=−16 or y=4
And x=16y2
⇒x=16 or x=1
Thus, the points are (1,4) and (16,−16), but (1,4) is the given point P.
⇒Q=(16,−16)
The distance between the points ({x}_{1},{y}_{1})&({x}_{2},{y}_{2}) is (x1−x2)2+(y1−y2)2
So, the length of focal chord PQ is
PQ=(16−1)2+(−16−4)2
=225+400=625=25 units.