Guangming Primary School held a charity sale. One of the groups sold pens at a unit price of 9.5 yuan. When 60% of the pens were sold, there was still 84 yuan short of the cost price. When they were all sold out, they made a profit of 372 yuan. How much is the cost price of a pen?
光明小学举行义卖活动，其中一个小组卖钢笔，以9.5元的单价出售，卖出60%时，还差84元卖到成本价．全部卖光时，赚了372元，成本价一支钢笔多少元？
<gadget id="calculator">372 + 84</gadget>
<output>456</output>

<gadget id="calculator">60 / 100</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">456 / 9.5 / (2/5)</gadget>
<output>120</output>

<gadget id="calculator">372 / 120</gadget>
<output>31/10 = around 3.1</output>

<gadget id="calculator">9.5 - (31/10)</gadget>
<output>6.4</output>

<result>6.4</result>