On selling 17 “Lays” at Rs. 720, there is a loss equal to the cost price of 5 “Lays”. The cost price of a “Lays” is:

Let's solve the problem step by step:

Step 1: Let's assume the cost price of one "Lays" is x.

Step 2: According to the given information, selling 17 "Lays" at Rs. 720 results in a loss equal to the cost price of 5 "Lays".

Step 3: The selling price of 17 "Lays" is Rs. 720, which means the selling price of one "Lays" is 720/17.

Step 4: The loss on selling 17 "Lays" is equal to the cost price of 5 "Lays". So, the loss is 5x.

Step 5: The selling price of 17 "Lays" is equal to the cost price minus the loss. Therefore, we can write the equation as:

720/17 = x - 5x

Step 6: Simplifying the equation:

720/17 = (1 - 5)x

720/17 = -4x

Step 7: Dividing both sides of the equation by -4:

(720/17) / -4 = x

Step 8: Calculating the value of x:

x = - (720/17) / 4

x = - 180/17

Step 9: Simplifying the value of x:

x ≈ -10.588

Therefore, the cost price of one "Lays" is approximately Rs. -10.588.