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A number of 512 identical small spheres are cast from a sphere of radius 40 cm, with the total volume of the small spheres being equal to the volume of the larger sphere. The diameter (in cm) of each of the small spheres is:

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RRB ALP 2025 CBT 2 Wiremen Question Paper (28-Jul-2026) (Shift 2)
The correct answer is
10

Calculating Small Sphere Diameter from Large Sphere Volume

The problem involves finding the diameter of small spheres formed by recasting a larger sphere, where the total volume remains constant.

Volume Formulas

The volume of a sphere is given by the formula \(V = \frac{4}{3}\pi r^3\), where \(r\) is the radius.

Relating Volumes

Let \(R\) be the radius of the large sphere and \(r\) be the radius of each small sphere. Let \(n\) be the number of small spheres.

  • Radius of the large sphere, \(R = 40\) cm.
  • Number of small spheres, \(n = 512\).
  • Volume of the large sphere: \(V_{large} = \frac{4}{3}\pi R^3\).
  • Volume of one small sphere: \(V_{small} = \frac{4}{3}\pi r^3\).
  • Total volume of small spheres: \(V_{total\_small} = n \times V_{small} = 512 \times \frac{4}{3}\pi r^3\).

Equating Volumes and Solving for Radius

The problem states that the total volume of the small spheres equals the volume of the large sphere:

\( V_{total\_small} = V_{large} \)

\( 512 \times \frac{4}{3}\pi r^3 = \frac{4}{3}\pi R^3 \)

Cancel out the common terms (\(\frac{4}{3}\pi\)):

\( 512 \times r^3 = R^3 \)

Substitute the value of \(R = 40\) cm:

\( 512 \times r^3 = (40)^3 \)

Solve for \(r^3\):

\( r^3 = \frac{(40)^3}{512} \)

Since \(512 = 8^3\), we can write:

\( r^3 = \frac{(40)^3}{8^3} = \left(\frac{40}{8}\right)^3 = (5)^3 \)

Therefore, the radius of each small sphere is \(r = 5\) cm.

Calculating Diameter

The diameter (\(d\)) of a sphere is twice its radius (\(d = 2r\)).

\( d = 2 \times r = 2 \times 5 \text{ cm} = 10 \text{ cm} \)

The diameter of each small sphere is 10 cm.

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Similar Questions

  1. A cylindrical rod has an outer curved surface area of \(7500 \text{ cm}^2\). If the length of the rod is 92 cm, then the outer radius (in cm) of the rod, rounded off to two places of decimal, is:
    \(\left(\text{Take } \pi = \frac{22}{7}\right)\)
  2. The radii of the internal and external surfaces of a hollow spherical shell are 6 cm and 4 cm respectively. If it is melted and recast into a solid cylinder of height $\frac{8}{3}$ cm, find the diameter of the cylinder.
  3. The diameter of a copper sphere is 12 cm. The sphere is melted and is drawn into a long wire of uniform circular cross - section. If the length of the wire is 48 cm, find its diameter.
  4. A hemispherical depression is cut out from one face of a cubical wooden block such that the diameter $l$ cm of the hemisphere is equal to the edge of the cube. Determine the surface area of the remaining solid.

Important Questions from Mensuration 3D (Notes)

  1. The height of a cylinder is 14cm and its curved surface area is 264cm². The volume of the cyclinder (in cm³) is:
    ($\pi=\frac{22}{7}$)
  2. A cylindrical rod has an outer curved surface area of \(7500 \text{ cm}^2\). If the length of the rod is 92 cm, then the outer radius (in cm) of the rod, rounded off to two places of decimal, is:
    \(\left(\text{Take } \pi = \frac{22}{7}\right)\)
  3. There is a wooden block in the form of a cube whose each side is 8 meters long. 

    The maximum possible number of cylinders with a diameter of 1 meter and a height of 4 meters were cut from this block. The cylinders are to be painted at the rate of ₹14 per square meter.
     

    What is the total amount (in ₹) needed to paint all the cylinders if we paint the entire surface of each cylinder? (Take $\pi = \frac{22}{7}$)

  4. If the lateral surface area of a cylinder is $140.1 \text{ cm}^2$ and its height is $3 \text{ cm}$, then find its volume. (Use $\pi = 3.14$ and round off to two decimal places.)
  5. Six cubes each of side 4 cm, are placed adjacent to each other. Find the volume of the cuboid so formed.
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