The store buys a batch of balls at a cost of 1.50 yuan each, and sells them at 2.00 yuan each. When the store sells 20 leftover balls, the cost has been fully recovered and the profit is 50 yuan. Ask how many balls the store originally purchased?
商店购进一批皮球每只成本1.50元，出售时每只售价2.00元，当商店卖剩皮球20只时，成本已全部收回，并且盈利50元．问商店原购进皮球多少只？
<gadget id="calculator">20 * 1.5</gadget>
<output>30</output>

<gadget id="calculator">30 + 50</gadget>
<output>80</output>

<gadget id="calculator">2 - 1.5</gadget>
<output>0.5</output>

<gadget id="calculator">80 / 0.5</gadget>
<output>160</output>

<gadget id="calculator">160 + 20</gadget>
<output>180</output>

<result>180</result>