Solution:

The total number of designs that are equal is 6.

AOB= 360°/6 = 60°

28 cm = Radius of the cover

₹ 0.35 per cm² = Cost of making design

The two arms of the triangle are equal since they are the radii of the circle, and one angle is 60°.

The triangle AOB is an equilateral triangle. As a result, area of triangle AOB = (√3/4)×a² sq. units

Here, a = OA

Therefore the area of equilateral ΔAOB = (√3/4)×28² = 333.2 cm²

The area of sector ACB = (60°/360°)×πr² cm² = 410.66 cm²

So, area of sector ACB – area of ΔAOB = Area of a single design

= 410.66 cm² – 333.2 cm² = 77.46 cm²

Therefore, the area of 6 designs = 6×77.46 cm² = 464.76 cm²

As a result, the total cost of making design = 464.76 cm² ×Rs.0.35 per cm²

= Rs. 162.66