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Question

A solid metal sphere is melted and smaller spheres of equal radii are formed. 5% of the volume of the sphere is lost during the process. The smaller sphere has a radius that is one-tenth of the large sphere. If 14 liters of paint was needed to paint the larger sphere, how many liters is needed to paint all the smaller spheres?

This question was previously asked in
RRB ALP 2025 CBT 2 Wiremen Question Paper (28-Jul-2026) (Shift 2)
The correct answer is
133 liters

The problem involves calculating the amount of paint needed for several smaller spheres formed by melting a larger sphere, considering volume loss and changes in radius.

Relating Surface Areas of Spheres

Let the radius of the large sphere be R and the radius of each smaller sphere be r. We are given that r = R/10.

The surface area of a sphere is given by the formula A = 4π * radius2.

  • Surface area of the large sphere: Alarge = 4πR2
  • Surface area of one small sphere: Asmall = 4πr2

The ratio of the surface area of a small sphere to the large sphere is:

\( \frac{A_{small}}{A_{large}} = \frac{4\pi r^2}{4\pi R^2} = \left(\frac{r}{R}\right)^2 \)

Substituting r = R/10:

\( \frac{A_{small}}{A_{large}} = \left(\frac{R/10}{R}\right)^2 = \left(\frac{1}{10}\right)^2 = \frac{1}{100} \)

This means the surface area of one small sphere is 1/100th of the large sphere's surface area. Since paint usage is proportional to surface area, one small sphere needs 1/100th the paint of the large sphere.

Calculating the Number of Smaller Spheres

The volume of a sphere is given by the formula V = (4/3)π * radius3.

  • Volume of the large sphere: Vlarge = (4/3)πR3
  • Volume of one small sphere: Vsmall = (4/3)πr3

The ratio of the volumes is:

\( \frac{V_{small}}{V_{large}} = \frac{(4/3)\pi r^3}{(4/3)\pi R^3} = \left(\frac{r}{R}\right)^3 \)

Substituting r = R/10:

\( \frac{V_{small}}{V_{large}} = \left(\frac{1}{10}\right)^3 = \frac{1}{1000} \)

So, the volume of one small sphere is 1/1000th of the large sphere's volume.

During melting, 5% of the volume is lost. This means 95% of the large sphere's volume is available to form smaller spheres.

Volume available = 0.95 * Vlarge.

The number of smaller spheres (N) is:

\( N = \frac{\text{Volume available}}{V_{small}} = \frac{0.95 \times V_{large}}{V_{large} / 1000} = 0.95 \times 1000 = 950 \)

Therefore, 950 smaller spheres are formed.

Total Paint Calculation

Paint needed for the large sphere = 14 liters. This corresponds to Alarge.

Paint needed for one small sphere corresponds to Asmall.

Paint per small sphere = 14 liters * (Asmall / Alarge) = 14 * (1/100) = 0.14 liters.

Total paint needed for all 950 smaller spheres:

Total Paint = Number of small spheres * Paint per small sphere

Total Paint = 950 * 0.14 liters

Total Paint = 950 * (14 / 100) liters

Total Paint = 9.5 * 14 liters

Total Paint = 133 liters.

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Similar Questions

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Important Questions from 3-D Mensuration

  1. Find the total surface area of a closed cylinder having a base radius of 70 m and a height of 110 m. [Use π = \(22\over7\)]

  2. Which of the following is a geometrical figure with a three-dimensional geometry that has eight vertices and six rectangular faces?

  3. The curved surface area of a cylinder is half of its total surface area. If its height is 195 cm, then find its diameter (in cm).
  4. The diameter of the base and slant height of a right circular cone are 30 cm and 113 cm, respectively. Find the volume (in cm³) of the given cone.
    (Use $\pi = \frac{22}{7}$)

  5. A cylindrical rod has an curved surface area of $4,900 \text{ cm}^2$. If the length of the rod is 97 cm, then the radius (in cm) of the rod, correct to two places of decimal, is:
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