A vendor bought toffees at 6 for a rupee. How many for a rupee must he sell to gain 20%?

The correct answer is (C)“5”.
Step-by-step Solution:
According to the problem statement,
The cost price (CP) of 6 toffees = Rs. 1
So according to the unitary method,

The cost price of 1 toffee = RS. \frac{1}{6}

Now we will use this formula to gain 20%

=\frac{\text { (selling price – cost price) }}{\text { cost price }} \times 100

1 toffee SP will be
20=\frac{\left(\text { selling price }-\frac{1}{6}\right)}{\frac{1}{6}} \times 100

\frac{20}{100} \times \frac{1}{6}=\text { (selling price of } 1 \text { toffee) }-\frac{1}{6}

=\frac{1}{5} \times \frac{1}{6}=(\text { SP of } 1 \text { toffee })-\frac{1}{6}

=\frac{1}{5} \times \frac{1}{6}+\frac{1}{6}=\text { (SP of } 1 \text { toffee) }

=\frac{1}{6}\left(1+\frac{1}{5}\right)=(\text { SP of } 1 \text { toffee })

=(\text { SP of } 1 \text { toffee })=\frac{1}{6} \times\left(\frac{6}{5}\right)
So,
The selling price (SP) of 1 toffee is RS. \frac{1}{5}

∴ According to the unitary method, 5 toffees selling price will be = 1

The vendor must sell 5 toffees for a rupee to gain 20%.