Q155
ArithmeticGeometry & Mensuration
The external diameter of a conical-coronet made off thermocol is 21 cm in length. To wrap up the outer surface of the coronet with foil, the expenditure will be ₹ 57·75 at the rate of 10p per cm². The height of the coronet is
- a.14 cm
- b.28 cm
- c.7 cm
- d.10 cm
Answer: (A) 14 cm
Foil area = 57·75 ÷ 0·10 = 577·5 cm², which is the curved surface $\pi r l$ with r = 10·5 cm, so $l = 577.5 \div 33 = 17.5$ cm. Height = $\sqrt{17.5^2 - 10.5^2} = \sqrt{196} = 14$ cm. 28 cm and 7 cm are double and half of this.