# Ex.11.3 Q8 Perimeter and Area - NCERT Maths Class 7

## Question

Find the cost of polishing a circular table-top of diameter \(1.6\rm\, m,\) if the rate of polishing is \(\rm Rs.\, 15/\rm\,m^2.\) (Take \(π = 3.14\))

## Text Solution

**What is known?**

Diameter of the circular table-top and the rate of polishing per meter.

**What is unknown?**

The cost of polishing a circular tabletop.

**Reasoning:**

To find the cost of polishing, first find out the area of the tabletop. To find the total cost of polishing, multiply the total area by rate of polishing per meter \((\rm Rs \,15).\)

**Steps:**

Diameter of the circular tabletop \(= 1.6\rm\, m\)

Radius of the circular tabletop

\(\begin{align}=\frac{{1.6}}{2}\end{align}\)\(= 0.8\rm\, m\)

Area of circular table top

\[\begin{align}&=\pi {r^2}\\ &= 3.14 \times 0.8 \times 0.8\\ &= 2.0096\;\rm{m^2}\end{align}\]

Since, the cost of \(1\,\rm m^2\) polishing \(= ₹\,15\)

\(\therefore\) the cost of polishing

\(2.0096 \rm\,m^2 = 15 \times 2.0096 = ₹ \,30.14\)

Thus, the cost of polishing a circular tabletop is \(₹ \,30.14\)