A vendor bought toffees at 6 for a rupee. How many for a rupee must he sell to gain20%?

Step 1: Determine the cost price of each toffee for the vendor. Since the vendor bought 6 toffees for a rupee, the cost price of each toffee is 1/6 = 0.167 rupees.

Step 2: Calculate the selling price for each toffee to gain a profit of 20%. The formula to calculate the selling price is: Selling Price = Cost Price + (Cost Price * Profit % / 100). Substituting the given values, we get Selling Price = 0.167 + (0.167 * 20 / 100) = 0.167 + 0.0334 = 0.2004 rupees.

Step 3: Determine how many toffees the vendor must sell for a rupee to achieve this selling price. Since 1 rupee is equal to 0.2004 rupees per toffee, the vendor must sell 1 / 0.2004 = 4.99, approximately 5 toffees for a rupee to gain a profit of 20%.