First, let's draw a sketch of the problem to better understand it:
Since the distances 20, 4√7 and x create a right triangle, we can use the Pythagorean theorem to calculate the value of x.
The Pythagorean theorem states that the length of the hypotenuse squared is equal to the sum of each leg squared.
So we have:
[tex]\begin{gathered} 20^2=(4\sqrt[]{7})^2+x^2 \\ 400=16\cdot7+x^2 \\ 400=112+x^2 \\ x^2=400-112 \\ x^2=288 \\ x=\sqrt[]{288}=\sqrt[]{2\cdot12\cdot12}=12\sqrt[]{2}\text{ ft} \end{gathered}[/tex]Therefore the wanted distance is 12√2 feet (16.97 ft).
