Explanation:
imagine this is the trigonometric triangle in a circle.
the center of the circle is the corner of angle 55°.
and the radius of the circle is the side of length 20.
so,
x = cos(55) × 20
because sine, cosine, ... are defined for the norm circle with radius 1, the corresponding lengths in a larger circle are the basic sine, cosine, ... functions multiplied by the actual radius.
x = cos(55) × 20 = 11.47152873... ≈ 11.47 units