Calculate the Original Cost of a Sofa Set Before Sales Tax

Explanation:

To find the original cost of the sofa set without the sales tax, we need to work backwards from the total cost, which includes the sales tax.

If the cost of a sofa inclusive of sales tax is Rs 10800, and the rate of sales tax is 20%, this means that the original cost plus 20% of it equals Rs 10800. Let's denote the original cost as 'C'. The equation can be set up as:

C + (20% of C) = Rs 10800

Convert the percentage to a decimal: 20% = 0.2

Then incorporate this into the equation: C + (0.20 × C) = Rs 10800

Combine like terms: 1.20 × C = Rs 10800

Now, solve for 'C' by dividing both sides by 1.20: C = Rs 10800 / 1.20 C = Rs 900

Therefore, the original cost of the sofa set before the sales tax was added is Rs 9000, which corresponds to option b in the question.

To find the original cost of the sofa set, you need to subtract the sales tax from the total cost. The sales tax is 20% of the original cost, so it can be calculated by dividing the total cost by 1.2. In this case, the total cost is Rs 10800, so the original cost would be Rs 10800 ÷ 1.2 = Rs 9000.