Find the cost price of 1 toffee:

10 toffees = Rs 3

1 toffee = 3 ÷ 10 = Rs 0.30

Find the selling price of 1 toffee:

6 toffees - Rs 3

1 toffee = 3 + 6 = Rs 0.375

Find the gain:

Gain = Selling Price - Cost Price

Gain = 0.375 0.3 = Rs 0.075

Find the gain percentage:

Gain Percentage = gain/cost price x 100

Gain Percentage = 0.075/0.3 x 100 = 25%

Given:
A man buys erasers at 10 for Rs. 3 and sells them at 6 for Rs. 3.

To find:
The gain percentage of the man.

Solution:

Step 1: Calculate the cost price per eraser
To find the cost price per eraser, we divide the total cost by the total number of erasers bought.

Cost of 10 erasers = Rs. 3
Cost of 1 eraser = Rs. 3 / 10 = Rs. 0.3

Therefore, the cost price per eraser is Rs. 0.3.

Step 2: Calculate the selling price per eraser
To find the selling price per eraser, we divide the total selling price by the total number of erasers sold.

Selling price of 6 erasers = Rs. 3
Selling price of 1 eraser = Rs. 3 / 6 = Rs. 0.5

Therefore, the selling price per eraser is Rs. 0.5.

Step 3: Calculate the profit per eraser
To find the profit per eraser, we subtract the cost price from the selling price.

Profit per eraser = Selling price per eraser - Cost price per eraser
= Rs. 0.5 - Rs. 0.3
= Rs. 0.2

Therefore, the profit per eraser is Rs. 0.2.

Step 4: Calculate the gain percentage
The gain percentage is calculated using the formula:

Gain percentage = (Profit / Cost price) * 100

In this case, the profit is Rs. 0.2 and the cost price is Rs. 0.3.

Gain percentage = (0.2 / 0.3) * 100
= 66.67%

Therefore, the gain percentage of the man is 66.67%.

Conclusion:
The man has a gain percentage of 66.67% on selling the erasers.