 
 <translate> 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. <hau> Bari p ya zama farashin cikakken alkalami. Mun san cewa an sayar da alkalami 10 na farko a farashin yau da kullum, don haka suna biyan 10 * p. Mun kuma san duk wani alkalami da aka saya bayan na farko 10 ne rabin farashin, don haka suka kudin (20 alkalami - 10 alkalami) * p / 2 a kan waɗanda. Idan muka ƙara farashin na farko 10 alkalami da alkalami sayar a rabin farashin shi daidai da jimlar sayan farashin $ 30 = 10 * p + (20 - 10) * p / 2. Saboda haka, $30 = 10 * p + 10 * p / 2. Kuma, $30 = 10 * p + 5 * p. Don haka, $30 = 15 * p. A ƙarshe, p = $2. Saboda haka, amsar ita ce 2. 