 
 <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. <ibo> Ka p bụrụ ego ole a na-akwụ n'ụkpụrụ pen. Anyị maara na a na-ere mkpịsị 10 mbụ na ọnụahịa nkịtị, yabụ ha na-eri 10 * p. Anyị makwaara na mkpịsị akwụkwọ ọ bụla a zụtara mgbe mbụ 10 bụ ọkara ọnụahịa, yabụ ha na-eri (20 mkpịsị akwụkwọ - 10 mkpịsị akwụkwọ) * p / 2 na ndị. Ọ bụrụ na anyị agbakwunye ọnụahịa nke mkpịsị akwụkwọ 10 mbụ na mkpịsị akwụkwọ ndị e rere na ọkara ọnụahịa, ọ ga-eme ka ọnụahịa ịzụta nke $30 = 10 * p + (20 - 10) * p / 2. N'ihi ya, $30 = 10*p + 10*p/2. Na, $30 = 10 * p + 5 * p. Ya mere, $30 = 15 * p. N'ikpeazụ, p = $2. Ya mere, azịza ya bụ 2. 