Each box of milk is a yuan, buy 6 boxes of milk, pay the cashier 400 yuan, how much should be returned.
每箱牛奶a元，买6箱牛奶，付给收银员400元，应找回多少元．
<gadget id="calculator">400 - 6</gadget>
<output>394</output>

<result>394</result>