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?
Step-by-step reasoning process below:
2
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.

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The answer: 2