The store bought some baseballs, 15 yuan each, and 20 yuan each for retail sale. When there are only 20 left, they will be sold at a 10% discount. After this, the total net profit is 860 yuan. How many baseballs did the store buy?
商店进了一些棒球，每个15元，零卖20元一个，当卖到只剩20个时，就打9折销售，这样卖完后，一共净赚860元，商店进了多少个棒球？
<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">20 * (9/10)</gadget>
<output>18</output>

<gadget id="calculator">18 - 15</gadget>
<output>3</output>

<gadget id="calculator">3 * 20</gadget>
<output>60</output>

<gadget id="calculator">860 - 60</gadget>
<output>800</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<gadget id="calculator">800 / 5</gadget>
<output>160</output>

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

<result>180</result>