If you solve the equations in physics for the radius of curvature, you find it is
r = v²/(g·tan(θ)) . . . . where g is the acceleration due to gravity, 9.8 m/s²
The required turn radius is
r = (23.7 m/s)²/(9.8 m/s²·tan(26.5°)) ≈ 114.96 m
The minimum distance she can begin her turn is 115 m.
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This question is in the math section, so we presume the required formula and its constants are known. If this were in the physics section, we might show the derivation of the formula based on the free body diagram of the jet ski, the centripetal acceleration, and the horizontal force vector resulting from the bank angle.
