Answer
$7263.1$ miles
Work Step by Step
Consider $r$ to be the radius of the tropic of cancer. Let the radius of the Earth be $R$.
Then, $\dfrac{r}{R}=\cos \theta \implies R \cos \theta=r$
Since, the circumference of the tropic of cancer is $2 \pi r$.
Then, $2 \pi r=2 \pi R \cos \theta \implies 2 \pi (3960) (\cos 23.5)=22817.75$ mile
Here, the diameter of the tropic of cancer is equal to $2r$.
and $2 r= 2R \cos \theta \implies 2 (3960) (\cos 23.5^{\circ})=7263.1$ miles
