From the drawing made, we can see that a triangle with a right angle is form, thus, a right triangle.
Since this is a right triangle, the properties of sine, cossine and tangent apply. RS is the leg opposite to the 80° angle, while 20m is the length of the hypotenuse. So we can use sine to find RS.
[tex]\begin{gathered} \sin 80\degree=\frac{RS}{ST}=\frac{RS}{20} \\ RS=20\sin 80\degree\approx20\cdot0.9848=19.696\approx19.7 \end{gathered}[/tex]
So, the closest alternative is 19.7 m.
