The school originally had 8 balls and 20 shuttlecocks. If the number of shuttlecocks is to be three times that of balls, how many more shuttlecocks should the school buy?
学校原有8个皮球，20个羽毛球，如果要使羽毛球的个数是皮球的3倍，学校还要再买多少个羽毛球．
<gadget id="calculator">8 * 3</gadget>
<output>24</output>

<gadget id="calculator">24 - 20</gadget>
<output>4</output>

<result>4</result>