A pen is priced at 30 yuan, and after selling it with a 20% discount, you can still get a profit of 4 yuan. What is the purchase price of this pen?
一支钢笔定价30元，打八折销售后仍可获得4元利润，这支钢笔的进价是多少元．
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">30 * (4/5)</gadget>
<output>24</output>

<gadget id="calculator">24 - 4</gadget>
<output>20</output>

<result>20</result>