31. 20 pens were bought at \( \$ 2 \) each and sold at 2 for \( \$ 5 \). What is the percentage profit?

To find the percentage profit, we first need to determine the total cost and the total revenue. The cost of 20 pens at \( \$ 2 \) each is: \[ 20 \times 2 = \$ 40 \] When selling the pens at 2 for \( \$ 5 \), the revenue from selling all 20 pens (which is 10 pairs) is: \[ 10 \times 5 = \$ 50 \] Now, we can find the profit by subtracting the total cost from the total revenue: \[ \text{Profit} = \text{Revenue} - \text{Cost} = 50 - 40 = \$ 10 \] Finally, to find the percentage profit: \[ \text{Percentage Profit} = \left( \frac{\text{Profit}}{\text{Cost}} \right) \times 100 = \left( \frac{10}{40} \right) \times 100 = 25\% \] So, the percentage profit is \( 25\% \).