A self-employed person buys a batch of balls at the wholesale price. The cost price is 0.15 yuan each, and the retail price is 0.25 yuan each. How many balls did this business buy in total?
某个体户按批发价买进一批皮球，成本价每个0.15元，零售价每个0.25元，当他卖到只剩20个皮球时，计算卖得得钱除去全部成本外获利10元，这个体户共买进皮球多少个？
<gadget id="calculator">0.25 - 0.15</gadget>
<output>0.1</output>

<gadget id="calculator">10 / 0.1</gadget>
<output>100</output>

<gadget id="calculator">100 + 20</gadget>
<output>120</output>

<result>120</result>