In order to promote milk, Hualian Supermarket launched a promotion for buying 4 boxes and getting 1 box free. It used to be 45 yuan per box, but grandma bought 4 boxes at a time. How much is each box cheaper?
为了促销牛奶，华联超市推出买4箱送1箱的优惠活动，原来每箱45元，奶奶一次买了4箱，每箱便宜多少元？
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">45 * (4/5)</gadget>
<output>36</output>

<gadget id="calculator">45 - 36</gadget>
<output>9</output>

<result>9</result>