Calculation of Closing Stock:

We can calculate the closing stock by using the following formula:

Closing Stock = Opening Stock + Purchases - Cost of Goods Sold

Here, we need to calculate the Cost of Goods Sold (COGS) which can be calculated as follows:

COGS = (Cash Sales + Credit Sales) x (100/103)

Now, substituting the given values in the above formulas, we get:

COGS = (60000 + 40000) x (100/103) = 93133.98

Closing Stock = 20000 + 70000 - 93133.98 = 16866.02

Therefore, the closing stock is Rs. 16866.02

Explanation of Gross Profit on Cost:

Gross Profit on Cost is the percentage of profit earned on the cost of goods sold. It is calculated by dividing the Gross Profit by the Cost of Goods Sold and multiplying the result by 100. The formula for Gross Profit on Cost is:

Gross Profit on Cost = (Gross Profit / Cost of Goods Sold) x 100

In this question, the rate of Gross Profit on Cost is given as 100/3%. This means that the Gross Profit earned on the Cost of Goods Sold is 100/3% of the Cost of Goods Sold.

To calculate the Gross Profit, we need to subtract the Cost of Goods Sold from the Total Sales. Then, we can use the above formula to calculate the Gross Profit on Cost.

We can also use the following formula to calculate the Gross Profit directly:

Gross Profit = Total Sales - Cost of Goods Sold

Once we have calculated the Gross Profit, we can use the above formula to calculate the Gross Profit on Cost.

Overall, Gross Profit on Cost is an important metric for businesses as it helps them understand the profitability of their operations. It is also used to compare the performance of different businesses in the same industry.