[tex]The\ circumference\ of\ a\ circle:C=2\pi r\ \ \ \ \ \ (r-a\ radius)\\\\C=15.7\to2\pi r=15.7\ \ \ \ |divide\ both\ sides\ by\ 2\\\\\pi r=7.85\ \ \ \ |divide\ both\ sides\ by\ \pi\\\\r=\frac{7.85}{\pi}\ (in)\\\\\pi\approx3.14\to r\approx\frac{7.85}{3.14}=\boxed{2.5\ (in)}[/tex]
