The stationery store buys 100 pens at a price of 4 yuan each, and sells each pen at a price of 6 yuan. When a part of the pens are sold, the remaining pens are sold at a 10% discount. When all the pens are sold, the store makes a total profit of 188 yuan. How many pens are there for 10% off?
文具店以每枝4元的价格进货100枝钢笔，卖出时每支标价6元，当卖了一部分钢笔后，剩余的钢笔打九折出售，钢笔全部卖出时商店共盈利188元．其中打九折出售的钢笔有几枝？
<gadget id="calculator">100 * 6</gadget>
<output>600</output>

<gadget id="calculator">100 * 4</gadget>
<output>400</output>

<gadget id="calculator">600 - 400 - 188</gadget>
<output>12</output>

<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">6 * (9/10)</gadget>
<output>27/5 = around 5.4</output>

<gadget id="calculator">4 - 4 - (27/5)</gadget>
<output>-27/5 = around -5.4</output>

<gadget id="calculator">6 + (-27/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">12 / (3/5)</gadget>
<output>20</output>

<result>20</result>