Every time a certain pencil case on the stationery cabinet is half sold, 15 supplements are transferred from the warehouse. After selling half of the eighth time, there are exactly 15 left. What is the original number of such pencil cases in the stationery cabinet?
文具柜上的某种笔盒每次卖出一半时，就从仓库中调来15个补充．到第八次卖出一半后，恰好余下15个．文具柜原有这种笔盒的个数是多少．
<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<result>30</result>