The stationery store buys a batch of pens at a wholesale price of 4 yuan each, and sells them at 7 yuan each. When (1/3) of this batch of pens is left, in addition to recovering the cost, it also earns 30 yuan. How many pens are there?
文具店按每支4元的批发价购进一批钢笔，按每支7元出售，当还剩这批笔的(1/3)时，除已收回成本外，还赚了30元，这批钢笔有多少支．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">7 * (1/3)</gadget>
<output>7/3 = around 2.333333</output>

<gadget id="calculator">7 - (7/3) - 4</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">30 / (2/3)</gadget>
<output>45</output>

<result>45</result>