Let the radius of circular field = r m. Speed of person in m/s =\( \Large \frac{30}{60}=\frac{1}{2} \)m/s According to the question, \( \Large \frac{2\pi r}{1/2}-\frac{2r}{1/2} \)=30 => \( \Large 4\pi r \)-4r=30 => \( \Large \left(4\times 22/7 - 4 \right)r \)=30 => (12.5-4)r=30 => (8.5)r=30 => r=\( \Large \frac{30}{8.5} \)=3.5 m.