The cost of painting a solid cone at 30 paise per cm2 is ₹ 660. find the radius of the cone if its slant height is 15 cm. (use π = )
Show answer & explanation
Correct answer: D
Explanation
The correct answer is option 4 i.e. 20 cm.
Cost = Rate × Total surface area
660 rupees = 66000 paise
TSA = 66000 / 30 = 2200 cm²
Total surface area of solid cone
TSA = π r (l + r)
2200 = 22/7 × r × (15 + r)
2200 × 7/22 = r(15 + r)
100 × 7 = r² + 15r
700 = r² + 15r
r² + 15r − 700 = 0
r² + 35r − 20r − 700 = 0
r(r + 35) − 20(r + 35) = 0
(r − 20)(r + 35) = 0
r = 20 cm