Given information:
- The man bought bananas at a rate of 20% per dozen.
- He sold the bananas at a rate of 10% from rupees 25.

Calculating the cost price:
To find the cost price of the bananas, we need to calculate the cost per dozen.

Step 1:
Calculate 20% of rupees 25 to find the cost of 1 dozen bananas:
20% of 25 = (20/100) * 25 = 5

Step 2:
Since the cost is given for 1 dozen bananas, we can say that the cost per dozen is rupees 5.

Step 3:
Now, let's calculate the cost price of the bananas.
If the cost per dozen is rupees 5, then the cost of 1 banana can be calculated as 5/12.

Step 4:
So, the cost price of 1 banana is rupees (5/12).

Calculating the selling price:
To find the selling price of the bananas, we need to calculate the selling price per banana.

Step 1:
Calculate 10% of rupees 25 to find the selling price of 1 banana:
10% of 25 = (10/100) * 25 = 2.5

Step 2:
Since the selling price is given for 1 banana, the selling price per banana is rupees 2.5.

Calculating gain or loss percentage:
To find the gain or loss percentage, we need to compare the cost price and the selling price.

Step 1:
Calculate the gain or loss by subtracting the cost price from the selling price:
Gain/Loss = Selling Price - Cost Price

Step 2:
Substitute the values:
Gain/Loss = 2.5 - (5/12)

Step 3:
Simplify the expression:
Gain/Loss = (30/12) - (5/12) = 25/12

Step 4:
To find the gain or loss percentage, divide the gain/loss by the cost price and multiply by 100:
Gain/Loss Percentage = (Gain/Loss / Cost Price) * 100

Step 5:
Substitute the values:
Gain/Loss Percentage = ((25/12) / (5/12)) * 100

Step 6:
Simplify the expression:
Gain/Loss Percentage = (25/5) * 100 = 500%

Conclusion:
The man has a gain of 500%.