The distance walkin is the circumference of the semicircle:
[tex]\begin{gathered} C=\pi\cdot r \\ \\ C=\pi\cdot3mi \\ \\ C=3\pi mi \end{gathered}[/tex]At a rate of 4 mi/h the woman will arrive at point C after 2.36 hours
[tex]3\pi mi\cdot\frac{1h}{4mi}=2.36h[/tex]The distance in a boat is the diameter of the lake:
[tex]\begin{gathered} d=2r \\ \\ d=2(3mi) \\ \\ d=6mi \end{gathered}[/tex]At a rate of 2 mi/h the woman will arrive at point C after 3 hours
[tex]6mi\cdot\frac{1h}{2mi}=3h[/tex]Then, she should go walking as the time to arrive at point C walking is 2.36 hours (shorter than the 3 hours in a boat)
