Guangming Store purchased a batch of pens at a cost of 7.5 yuan each. Now sell at a price of 10.5 each, and when there are 6 left after selling some, not only will all the costs be recovered, but a profit of 57 yuan will also be made. How many pens are there in total?
光明商店以每支成本7.5元的价格购进一批钢笔．现在以每支10.5的价格进行出售，卖出一些后还剩6支时，此时不但全部收回成本还盈利57元．这批钢笔一共多少支？
<gadget id="calculator">10.5 * 6</gadget>
<output>63</output>

<gadget id="calculator">63 + 57</gadget>
<output>120</output>

<gadget id="calculator">10.5 - 7.5</gadget>
<output>3</output>

<gadget id="calculator">120 / 3</gadget>
<output>40</output>

<result>40</result>