**Profit Percentage by Ratio Method**

To find the profit percentage by ratio method, we need to consider two factors: the selling price and the cost price. In this case, the selling price is given as a 10% gain, and the weight of the rice is 20% less than the original weight. Let's break down the solution into steps:

**Step 1: Understand the Given Information**
- Selling price gain: 10%
- Weight of the rice: 20% less than the original weight

**Step 2: Assume a Base Value**
Let's assume the original weight of the rice to be 100 units. This is just an arbitrary value to make calculations easier, and the ratio method allows us to work with any base value.

**Step 3: Calculate the Selling Price**
Since the selling price gain is given as 10%, we can calculate the selling price using the formula:
Selling Price = Cost Price + Gain
Selling Price = 100 + (10/100) * 100
Selling Price = 100 + 10
Selling Price = 110 units

**Step 4: Calculate the Cost Price**
To calculate the cost price, we need to consider that the weight of the rice is 20% less than the original weight. So, if the original weight is 100 units, the weight of the rice in this case would be:
Weight of the Rice = Original Weight - (20/100) * Original Weight
Weight of the Rice = 100 - (20/100) * 100
Weight of the Rice = 100 - 20
Weight of the Rice = 80 units

**Step 5: Calculate the Profit**
Profit = Selling Price - Cost Price
Profit = 110 - 80
Profit = 30 units

**Step 6: Calculate the Profit Percentage**
Profit Percentage = (Profit / Cost Price) * 100
Profit Percentage = (30 / 80) * 100
Profit Percentage = 37.5%

Therefore, the profit percentage, by ratio method, is 37.5%.