To celebrate Children's Day, Xinhua Bookstore is offering to buy 4 sets and get 1 free. It is known that each set is 45 yuan. If you buy 5 sets at a time, how much is each set cheaper?
新华书店为庆祝儿童节，买4套送1套，已知原来每套45元，一次买5套，每套便宜多少钱？
<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>