Explanation:
To calculate the gain percent of the shopkeeper, we need to first understand the concept of cost price, selling price, and profit.

Cost Price: The amount paid by the shopkeeper to purchase the goods from the supplier is called the cost price (CP).

Selling Price: The amount at which the shopkeeper sells the goods to the customer is called the selling price (SP).

Profit: The difference between the selling price and the cost price is called the profit (P).

Now, let's calculate the gain percent of the shopkeeper.

Step 1: Marking the Goods 20% Above the Cost Price
If the cost price of the goods is CP, then the shopkeeper marks the goods 20% above the cost price. This means that the selling price of the goods is:

SP1 = CP + 20% of CP
SP1 = CP + 0.2CP
SP1 = 1.2CP

Step 2: Giving a Discount of 10%
Now, the shopkeeper gives a discount of 10% on the selling price of the goods. This means that the new selling price of the goods is:

SP2 = SP1 - 10% of SP1
SP2 = 1.2CP - 0.1(1.2CP)
SP2 = 1.08CP

Step 3: Calculating the Profit
The profit earned by the shopkeeper is the difference between the selling price and the cost price. Therefore, the profit is:

P = SP2 - CP
P = 1.08CP - CP
P = 0.08CP

Step 4: Calculating the Gain Percent
The gain percent of the shopkeeper is the profit earned as a percentage of the cost price. Therefore, the gain percent is:

Gain Percent = (Profit/Cost Price) x 100
Gain Percent = (0.08CP/CP) x 100
Gain Percent = 8%

Conclusion:
The gain percent of the shopkeeper is 8%. This means that the shopkeeper earns a profit of 8% on his cost price after marking the goods 20% above the cost price and giving a discount of 10%.

Let the earlier price be rs100.If 20 percent is increased then the cost price becomes 100+20=rs120. After that 10 percent discount is given on it. So, 10/100 of 120 is 12. Therefore to find the real selling price we will have to subtract 12 from 120 = 108So gain will be 108-100=8. And therefore gain percent will be 8 percent.