Xiaoqiang likes to collect stamps. He bought two kinds of stamps with 4 cents and 8 cents for 1 yuan, a total of 20 stamps. So how many stamps did he buy for 4 cents?
小强爱好集邮，他用1元钱买了4分和8分的两种邮票，共20张．那么他买了4分邮票多少张．
<gadget id="calculator">20 * 8</gadget>
<output>160</output>

<gadget id="calculator">1 * 100</gadget>
<output>100</output>

<gadget id="calculator">160 - 100</gadget>
<output>60</output>

<gadget id="calculator">8 - 4</gadget>
<output>4</output>

<gadget id="calculator">60 / 4</gadget>
<output>15</output>

<result>15</result>