Xinhua Bookstore launched a book promotion to celebrate Children's Day. Each set of children's encyclopedia is 40 yuan, buy 3 sets and get 1 set free, Beibei has to buy 4 sets at a time, how much is each set cheaper than before?
新华书店为庆祝六一儿童节开展图书大促俏．儿童百科每套40元，买3套送1套，贝贝一次要买4套，每套比原来便宜多少钱？
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">40 * (3/4)</gadget>
<output>30</output>

<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<result>10</result>