Mutton slices are 30 yuan per box, "buy four get one free". Grandma Li bought four boxes at a time, how much is each box cheaper?
羊肉片每盒30元，"买四送一"．李奶奶一次买了四盒，每盒便宜多少钱？
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">30 * (4/5)</gadget>
<output>24</output>

<gadget id="calculator">30 - 24</gadget>
<output>6</output>

<result>6</result>