Introduction:
The replacement of all existing taxes on good X by the proposed single Goods and Services Tax (GST) is expected to have a substantial effect on the overall tax burden on the good. This is likely to impact the price and quantity of the good. Let's explore the chain of effects in detail:

1. Elimination of Multiple Taxes:
The implementation of the single GST system would eliminate multiple taxes, such as excise duty, sales tax, and value-added tax, which were previously applicable to good X. This simplification would reduce the compliance burden on businesses and ensure a more transparent tax structure.

2. Input Tax Credit:
Under the GST regime, businesses would be able to claim input tax credit for the taxes paid on inputs or raw materials used in the production of good X. This would help in reducing the overall cost of production, as businesses can offset the tax paid on inputs against the tax liability on the final product.

3. Price Reduction:
With the elimination of multiple taxes and the availability of input tax credit, the cost of production for businesses would decrease. This reduction in the cost of production is likely to be passed on to the consumers in the form of lower prices for good X. Therefore, there would be a downward pressure on the price of the good.

4. Increase in Demand:
The decrease in price resulting from the implementation of the single GST is likely to increase the demand for good X. Lower prices make the good more affordable for consumers, leading to an increase in the quantity demanded. This can be attributed to the price elasticity of demand, where a decrease in price leads to a relatively larger increase in quantity demanded.

5. Economies of Scale:
The implementation of the single GST system would promote a unified market across the country, eliminating inter-state barriers and facilitating the movement of goods. This would encourage businesses to expand their operations and take advantage of economies of scale. Increased production and economies of scale can further contribute to lower production costs and potentially lead to a reduction in the price of good X.

Conclusion:
The replacement of all existing taxes on good X by the proposed single GST is expected to bring down the overall tax burden on the good. This reduction, coupled with input tax credit and economies of scale, is likely to result in lower prices for good X. The decrease in prices can lead to an increase in demand, ultimately affecting the quantity of the good. However, it's important to note that the actual impact on price and quantity would depend on various factors such as the elasticity of demand, market competition, and the behavior of businesses.