A park is in the form of a rectangle 120 m × 100 m.

Let the radius of circular lawn be r. Then,

Area of circular lawn $=\pi r^{2}$

It is given that

Area of park excluding lawn $=$ Area of rectangle-Area of circular lawn

$8700=120 \times 100-\pi r^{2}$

$\pi r^{2}=12000-8700$

$\frac{22}{7} r^{2}=3300$

$r^{2}=\frac{3300 \times 7}{22}$

$r^{2}=1050$

$r=\sqrt{1050}$

$r=32.40 \mathrm{~m}$

Hence, radius of circular lawn is $32.40 \mathrm{~m}$.