Xiao Ming bought X pencils with 20 yuan, each 3.6 yuan, how much money is left? Use a formula containing letters to express how many yuan it is.
小明用20元钱，买了X支铅笔，每支3.6元，还剩多少元？用含有字母的式子表示是多少元．
<gadget id="calculator">20 - 3.6</gadget>
<output>16.4</output>

<result>16.4</result>