Answer
$\text{approximately }51.2\text{ }ft$
Work Step by Step
Let $x$ be the width of the pond at the crossing point. Using $a^2+b^2=c^2$ or the Pythagorean Theorem, then
\begin{array}{l}\require{cancel}
40^2+x^2=65^2
.\end{array}
Using the properties of equality, the equation above is equivalent to
\begin{array}{l}\require{cancel}
x^2=65^2-40^2
\\
x^2=4225-1600
\\
x^2=2625
.\end{array}
Taking the square root of both sides, the equation above is equivalent to
\begin{array}{l}\require{cancel}
x=\sqrt{2625}
\\
x\approx51.2
.\end{array}
Hence, the width of the pond at the crossing point, $x,$ is $
\text{approximately }51.2\text{ }ft
.$
