8. A box of 30 pens costs \( \$ 80.00 \). Frank bought 3 boxes and sold the pens individually to make a profit of \( 50 \% \). For every 4 pens a customer purchased, he got one free. What was the selling price of 1 pen?

To find the selling price of one pen, we first calculate how much Frank spent on the pens. He bought 3 boxes of 30 pens each, totaling 90 pens for \( 3 \times 80 = \$240 \). To achieve a 50% profit, Frank needs to sell the pens for \( 240 \times 1.5 = \$360 \). Now, considering that for every 4 pens sold, the customer gets 1 free, each customer effectively receives 5 pens for the price of 4. Therefore, if he sells \( 90 \) pens, the number of sets of 5 he can provide is \( 90 / 5 = 18 \) sets. For these 18 sets, he will charge for only 4 pens per set, which means he'll sell \( 18 \times 4 = 72 \) pens. Now, to find the selling price per pen, we divide the total selling price by the total number of pens sold: \[ \text{Selling price per pen} = \frac{360}{72} = \$5. \] Thus, the selling price of one pen is **\$5.00**.