When the circumference of a toy ballon is increased from

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When the circumference of a toy ballon is increased from 20 cm to 25 cm, its radius (in cm) is increased by :

Here, we can treat the balloon as sphere.
Its circumference = 2πr
∴ 2πr₁ = 20 ...(i)
2πr₂ = 25 ....(ii)
On dividing equation (ii) by (i),

⇒	2πr₂	=	25	⇒	r₂	=	5
	2πr₁		20		r₁		4

∴ Increase = r₂ – r₁

=	5	r₁ - r₁	1	r₁
	4		4

=	1	×	20	=	5	[From(i)
	4		2π		2π