To solve the problem of finding the profit percentage when the cost price is 80% of the selling price, we can use the following steps:
\(\text{Profit} = \text{Selling Price} - \text{Cost Price} = S - 0.8 \times S = 0.2 \times S\)
\(\text{Profit Percentage} = \left( \frac{\text{Profit}}{\text{Cost Price}} \right) \times 100 = \left( \frac{0.2 \times S}{0.8 \times S} \right) \times 100\)
\(= \left( \frac{0.2}{0.8} \right) \times 100 = 0.25 \times 100 = 25\%\)