Beibei likes collecting stamps. She bought 20 stamps of 4 cents and 8 cents with 10 yuan, so how many stamps of 4 cents did she buy?
贝贝爱好集邮，她用10元钱买了4角和8角的两种邮票共20张，那么她买了4角的邮票多少张．
<gadget id="calculator">0.8 * 20</gadget>
<output>16</output>

<gadget id="calculator">16 - 10</gadget>
<output>6</output>

<gadget id="calculator">0.8 - 0.4</gadget>
<output>0.4</output>

<gadget id="calculator">6 / 0.4</gadget>
<output>15</output>

<result>15</result>