Dimensions of rectangular park length = 120m

Breath = 100m

Area of park = l x b

=120x100=12000m^{2}.

Let radius of circular lawn be r

Area of circular lawn = πr^{2}

Area of remaining park excluding lawn = (area of park) – (area of circular lawn)

⇒ 8700 = 12000 – πr^{2}

⇒ πr^{2} = 12000 – 8700 = 3300

⇒ 22/7 x r^{2}=3300

⇒ r^{2}=150x7=1050

⇒ r = √1050=5√42 metres

∴ radius of circular lawn = 5√42 metres.