Analyze Given Data

The vendor sells $10$ toffees for $1$ rupee and makes a $20\%$ profit.

$SP_{10} = \text{Re. } 1, \text{ Profit} = 20\%$

Find Selling Price of One Toffee

The selling price of $1$ toffee is calculated as $SP = \frac{1}{10}$ rupee.

$SP_1 = \frac{1}{10}$

Calculate Cost Price of One Toffee

Using the relation between Cost Price and Selling Price at $20\%$ profit: $CP_1 = \frac{SP_1}{1 + \text{Gain}\%/100}$.

$CP_1 = \frac{1/10}{1.2} = \frac{1}{12}$

Determine Number of Toffees Bought per Rupee

Since the cost price of $1$ toffee is $\frac{1}{12}$ rupee, the vendor bought $12$ toffees for $1$ rupee.

$\text{Number of toffees} = 12$