To celebrate Children's Day, Xinhua Bookstore offers children's encyclopedias for 45 yuan per set, buy 4 sets of children's encyclopedias and get 1 set free. Buy 5 sets at a time, how much is each set cheaper?
新华书店为庆祝"六一"儿童节，儿童百科每套45元，买4套儿童百科送1套．一次买5套，每套便宜多少钱？
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<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>