# Ex.7.1 Q4 Cubes and Cube Roots - NCERT Maths Class 8

## Question

Parikshit makes a cuboid of plasticine of sides \(5 \,\rm{cm},\) \(2 \,\rm{cm,}\) \(5\, \rm{cm.}\) How many such cuboids will he need to form a cube?

## Text Solution

**What is known?**

Dimensions of cuboid \(5\;{\rm{cm}}\, \times\,2\;{\rm{cm}}\,\times \,5\;\rm{cm}.\)

**What is unknown?**

To find out the number of cuboids to form a cube.

**Reasoning:**

Number of cuboids required \(\begin{align}=\frac{{{\text{Volume of cube}}}}{{{\text{Volume of cuboid}}}}\end{align}\)

**Steps:**

\[\begin{align}\text{Volume of cuboid} &= {\rm{length}} \times {\rm{breadth}} \times {\rm{height}}\\

&= 5 \times 2 \times 5\\&= {5^2} \times {2^1}{\rm{c}}{{\rm{m}}^3}\end{align}\]

To make the volume of cuboid as a cube number we need to multiply it by \(5 \times 2 \times 2\)

\[\begin{align}\text {Newly formed cube}&= {5^2} \times {2^1} \times 5\; \times 2 \times 2\\&= {5^3} \times {2^3}{\rm{c}}{{\rm{m}}^3}\end{align}\]

\[\begin{align}\text {Number of cuboids required}&=\frac{5^{3} \times 2^{3}}{5^{2} \times 2}\\&=\frac{5 \times \not5 \times \not5 \times \not2 \times 2 \times 2}{\not 5 \times \not 5 \times \not2}\end{align}\]

\(\therefore \;\) Number of cuboids required to make a cube \(=\) \(\begin{align}5\times 2\times 2=20\end{align}\)

\(20\) cuboids required.