In a circular sheet of paper of radius 20 cm , a sector of 30% area is removed and remaining part is used to make a conical surface . How to find the approximate volume of conical surface in cubic cm . 

Dear Student,

Circumference of circular sheet of paper = $2\pi r = 2\pi \times 20 = 40\pi$ cm
Perimeter of base of conical surface = $40\pi \times \frac{\left(100-30\right)}{100} = 40\pi \times \frac{70}{100} = 28\pi$ cm
Radius of base of conical surface = $\frac{28\pi }{2\pi }$ = 14 cm
Height of conical surface = $\sqrt{{20}^2-{14}^2} = \sqrt{400 - 196} = \sqrt{204} \approx 14.28$ cm
Volume of conical surface = $\frac{1}{3}\pi r^{2}h = \frac{1}{3}\pi \times \left({14}^2\right)\times 14.28 \approx 2929.4944 c{m}^3$
Regards,