Shortcut Trick
Total volume is conserved. Base area is Volume ÷ Height. The volume ratio 3:4 is extra information!
⇒ Old flat area (cylinder bases) = 2 × (Volume ÷ H) = 2V ÷ H
⇒ New flat area (cone bases) = 3 × (Volume ÷ h) = 3V ÷ h
⇒ Area Ratio (New : Old) = (3 ÷ h) : (2 ÷ H) = (3 ÷ 35) : (2 ÷ 20) = 60 : 70 = 6 : 7
⇒ Decrease in flat area = 7 − 6 = 1 part
⇒ Percentage decrease = (1 ÷ 7) × 100 = 142⁄7%
∴ The correct answer is 142⁄7% decrease.
Alternate Method
Given:
Cylinder: Radius (R) = 7 cm, Height (H) = 20 cm
Cones: Height (h) = 35 cm, Volume ratio = 3 : 4
R=7
H=20
Old Area = 2πR²
→
Melted
r₁
h=35
r₂
h=35
New Area = πr₁² + πr₂²
Volume Ratio 3 : 4
Step 1: Calculate Cylinder Volume
⇒ Total Volume (V) = πR2H = π × 72 × 20 = 980π cm3
Step 2: Calculate Cone Volumes
⇒ Total volume is distributed in 3 : 4 ratio (Sum of parts = 7).
⇒ V1 = (3 ÷ 7) × 980π = 420π cm3
⇒ V2 = (4 ÷ 7) × 980π = 560π cm3
Step 3: Calculate Base Areas (πr2) of Cones
⇒ V1 = 1⁄3 × Base Area1 × h ⇒ 420π = 1⁄3 × πr12 × 35
⇒ Base Area1 = πr12 = (420π × 3) ÷ 35 = 36π cm2
⇒ V2 = 1⁄3 × Base Area2 × h ⇒ 560π = 1⁄3 × πr22 × 35
⇒ Base Area2 = πr22 = (560π × 3) ÷ 35 = 48π cm2
Step 4: Calculate Percentage Change in Flat Area
⇒ Old Flat Area (Top + Bottom bases) = 2 × πR2 = 2 × 49π = 98π cm2
⇒ New Flat Area (Cone bases) = 36π + 48π = 84π cm2
⇒ Area Decrease = 98π − 84π = 14π cm2
⇒ Percentage Decrease = (14π ÷ 98π) × 100 = (1 ÷ 7) × 100 = 142⁄7%
∴ The correct answer is 142⁄7% decrease.
Additional Information
Recasting Solids Principle
When one 3D shape is melted to form another, the total volume strictly remains constant (V1 = V2), but the total surface area and flat surface area generally change depending on the new dimensions.
Flat Surface Area Geometry
For a solid cylinder, the flat surface consists of the top and bottom circular faces: Area = 2πr2. For a solid cone, the flat surface is only the bottom circular face: Area = πr2.
Volume and Base Area Relation
Both shapes have volumes proportional to their base areas. Cylinder: V = Base Area × Height. Cone: V = 1⁄3 × Base Area × Height. Using this relation allows direct calculation of base areas without computing actual radii.