From the given data,

The owner first lost Rs. 100 in cash, but after the thief bought Rs. 60 goods, he get back that Rs. 100 but he lost Rs. 60 value of goods and Rs. 40 in change.

The logic in the challenge is

Let the given be resembled in letters like R, G & B.

In the given puzzle, the logic is

The given math puzzle follows a logic that,

5 + 5 = 10

From the given data,

Here by observing the figure, 

This can be done in two ways

From the hints,

In the given puzzle,

Let the required number be 'N'

In the given puzzle, the logic behind is

From the given equations,

Here in the given puzzle,

Except 142 all are divisible by number '7'.

The puzzle's logic is

To get a pair of black rings 100 percent certain