A shop is offering a discount on pens. If you buy 10 pens at the regular price, you can get the next 10 for half off. A customer came in and bought 20 pens for $30. What's the regular price of one pen in dollars? Let's think first. Chain of thought:
Let p be the cost of a full - priced pen. We know that the first 10 pens are sold at the regular price, so they cost 10 * p. We also know the any pens purchased after the first 10 are half - price, so they cost (20 pens - 10 pens) * p / 2 on those. If we add the cost of the first 10 pens and the pens sold at half - price it equals the total purchase of price of $30 = 10 * p + (20 - 10) * p / 2. Thus, $30 = 10 * p + 10 * p / 2. And, $30 = 10 * p + 5 * p. So, $30 = 15 * p. Finally, p = $2.
Therefore, the answer is 2.