The maximum safe speed, in miles per hour, at which a car can travel on a curved road is given by $\displaystyle {V}{\left({r}\right)}=\sqrt{{{2.3}{r}}}$ where $\displaystyle {r}$ represents the radius of curvature, in feet. A highway crew measures the radius of curvature at an exit ramp on a highway as 390 feet. What is the maximum safe speed in miles per hour?

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